Update changelog for unreleased 16.0 changes (#10730)

* First shot at updating changelog for 16.0

**what is the change?:**
Added an 'unreleased' section to the changelog with info from #10294

**why make this change?:**
To get things set for the 16.0 release.

**test plan:**
Visual inspection

**issue:**
#8854

* Fix typos and formatting errors in changelog

* Add requestAnimationFrame and remove "New helpful warnings"

**what is the change?:**
In response to helpful code review -
- Add mention of dependency on `requestAnimationFrame` and need to
  polyfill that as well as `Map` and `Set`
- Remove "New helpful warnings" section; it was incomplete, and there
  are so many new and updated warnings that it might not be reasonable
  to cover them in the changelog.

**why make this change?:**
Accuracy

**test plan:**
Visual inspection

**issue:**
issue #8854

* Improve wording

* Improve wording and fix missing links

* Add backticks to file names & code; wording tweak

* Break "Major Changes" into "New Feature" and "Breaking Changes"

* Add server side render changes to 16.0 changelog

* Change gist link from mine to gaearons

* Add note about returning fragments

* fix misc nits

* Misc. formatting/wording fixes to changelog

**what is the change?:**
Thanks to the kind code review comments of @gaearon and @nhunzaker we
have
- removed the non-deterministic bold styling on some bullet points
- improved wording of the bullet points for portals, attribute whitelist
  changes, and server rendering changes
- Add note about error boundaries including a breaking change to error
  handling behavior.
- punctuation and capitalization fixes

**why make this change?:**
Clarity and correctness

**test plan:**
Visual inspection

**issue:**
#8854

* fix broken link

CHANGELOG.md

@@ -1,3 +1,53 @@
+## [Unreleased]
+<details>
+  <summary>
+    Changes that have landed in master but are not yet released.
+    Click to see more.
+  </summary>
+
+### New JS Environment Requirements
+
+ * React 16 depends on the collection types [Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) and [Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set), as well as [requestAnimationFrame](https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame). If you support older browsers and devices which may not yet provide these natively (e.g. <IE11), [you may want to include a polyfill](https://gist.github.com/gaearon/9a4d54653ae9c50af6c54b4e0e56b583).
+
+### New Features
+* Components can now return arrays and strings from `render`. (Docs coming soon!)
+* Improved error handling with introduction of "error boundaries". [Error boundaries](https://facebook.github.io/react/blog/2017/07/26/error-handling-in-react-16.html) are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
+* First-class support for declaratively rendering a subtree into another DOM node with `ReactDOM.createPortal()`. (Docs coming soon!)
+* Streaming mode for server side rendering is enabled with `ReactDOMServer.renderToNodeStream()` and `ReactDOMServer.renderToStaticNodeStream()`. ([@aickin](https://github.com/aickin) in [#10425](https://github.com/facebook/react/pull/10425), [#10044](https://github.com/facebook/react/pull/10044), [#10039](https://github.com/facebook/react/pull/10039), [#10024](https://github.com/facebook/react/pull/10024), [#9264](https://github.com/facebook/react/pull/9264), and others.)
+* [React DOM now allows passing non-standard attributes](https://facebook.github.io/react/blog/2017/09/08/dom-attributes-in-react-16.html). ([@nhunzaker](https://github.com/nhunzaker) in [#10385](https://github.com/facebook/react/pull/10385), [10564](https://github.com/facebook/react/pull/10564), [#10495](https://github.com/facebook/react/pull/10495) and others)
+
+### Breaking Changes
+- There are several changes to the behavior of scheduling and lifecycle methods:
+  * `ReactDOM.render()` and `ReactDOM.unstable_renderIntoContainer()` now return `null` if called from inside a lifecycle method.
+    * To work around this, you can either use [the new portal API](https://github.com/facebook/react/issues/10309#issuecomment-318433235) or [refs](https://github.com/facebook/react/issues/10309#issuecomment-318434635).
+  * Minor changes to `setState` behavior:
+    * Calling `setState` with null no longer triggers an update. This allows you to decide in an updater function if you want to re-render.
+    * Calling `setState` directly in render always causes an update. This was not previously the case. Regardless, you should not be calling `setState` from render.
+    * `setState` callback (second argument) now fires immediately after `componentDidMount` / `componentDidUpdate` instead of after all components have rendered.
+  * When replacing `<A />` with `<B />`,  `B.componentWillMount` now always happens before  `A.componentWillUnmount`. Previously, `A.componentWillUnmount` could fire first in some cases.
+  * Previously, changing the `ref` to a component would always detach the ref before that component's render is called. Now, we change the `ref` later, when applying the changes to the DOM.
+  * It is not safe to re-render into a container that was modified by something other than React. This worked previously in some cases but was never supported. We now emit a warning in this case. Instead you should clean up your component trees using `ReactDOM.unmountComponentAtNode`. [See this example.](https://github.com/facebook/react/issues/10294#issuecomment-318820987)
+  * `componentDidUpdate` lifecycle no longer receives `prevContext` param. ([@bvaughn](https://github.com/bvaughn) in [#8631](https://github.com/facebook/react/pull/8631))
+  * Shallow renderer no longer calls `componentDidUpdate()` because DOM refs are not available. This also makes it consistent with `componentDidMount()` (which does not get called in previous versions either).
+  * Shallow renderer does not implement `unstable_batchedUpdates()` anymore.
+- The names and paths to the single-file browser builds have changed to emphasize the difference between development and production builds. For example:
+  - `react/dist/react.js` → `react/umd/react.development.js`
+  - `react/dist/react.min.js` → `react/umd/react.production.min.js`
+  - `react-dom/dist/react-dom.js` → `react-dom/umd/react-dom.development.js`
+  - `react-dom/dist/react-dom.min.js` → `react-dom/umd/react-dom.production.min.js`
+* The server renderer has been completely rewritten, with some improvements:
+  * Server rendering does not use markup validation anymore, and instead tries its best to attach to existing DOM, warning about inconsistencies. It also doesn't use comments for empty components and data-reactid attributes on each node anymore.
+  * Hydrating a server rendered container now has an explicit API. Use `ReactDOM.hydrate` instead of `ReactDOM.render` if you're reviving server rendered HTML. Keep using `ReactDOM.render` if you're just doing client-side rendering.
+* When "unknown" props are passed to DOM components, for valid values, React will now render them in the DOM. [See this post for more details.](https://facebook.github.io/react/blog/2017/09/08/dom-attributes-in-react-16.html) ([@nhunzaker](https://github.com/nhunzaker) in [#10385](https://github.com/facebook/react/pull/10385), [10564](https://github.com/facebook/react/pull/10564), [#10495](https://github.com/facebook/react/pull/10495) and others)
+* Errors in the render and lifecycle methods now unmount the component tree by default. To prevent this, add [error boundaries](https://facebook.github.io/react/blog/2017/07/26/error-handling-in-react-16.html) to the appropriate places in the UI.
+
+### Removed Deprecations
+
+- There is no `react-with-addons.js` build anymore. All compatible addons are published separately on npm, and have single-file browser versions if you need them.
+- The deprecations introduced in 15.x have been removed from the core package. `React.createClass` is now available as create-react-class, `React.PropTypes` as prop-types, `React.DOM` as react-dom-factories, react-addons-test-utils as react-dom/test-utils, and shallow renderer as react-test-renderer/shallow. See [15.5.0](https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html) and [15.6.0](https://facebook.github.io/react/blog/2017/06/13/react-v15.6.0.html) blog posts for instructions on migrating code and automated codemods.
+
+</details>
+
 ## 15.6.1 (June 14, 2017)
 
 ### React DOM