Add 'Installation' and 'Changelog' to 15.6.0 blog post

**what is the change?:**
Added the 'Installation' section we have on most release blog posts,
customized for the 15.6.0 version of React.
Added the 'Changelog' from master to the blog post.

**why make this change?:**
To show folks how to install React and what changes are in this release.

**test plan:**
Visual inspection

**issue:**
facebook#9398

docs/_posts/2017-06-13-react-v15.6.0.md

@@ -2,6 +2,7 @@
 title: "React v15.6.0"
 author: flarnie
 ---
+
 Today we are releasing React 15.6.0. As we prepare for React 16.0, we have been fixing and cleaning up many things. This release continues to pave the way.
 
 ## Improving Inputs
@@ -22,3 +23,69 @@ Thanks to [Jason Quense](https://github.com/jquense) and everyone who helped out
 We are also including a couple of new warnings for upcoming deprecations. These should not affect most users, and for more details see the changelog below.
 
 After the last release, we got valuable community feedback that deprecation warnings were causing noise and failing tests. **In React 15.6, we have downgraded deprecation warnings to use 'console.warn' instead of 'console.error'.** Our other warnings will still use 'console.error' because they surface urgent issues which could lead to bugs. Unlike our other warnings, deprecation warnings can be fixed over time and won't cause problems in your app if shipped. We believe that downgrading the urgency of deprecation warnings will make your next update easier. Thanks to everyone who was involved in the discussion of this change.
+
+---
+
+## Installation
+
+We recommend using [Yarn](https://yarnpkg.com/) or [npm](https://www.npmjs.com/) for managing front-end dependencies. If you're new to package managers, the [Yarn documentation](https://yarnpkg.com/en/docs/getting-started) is a good place to get started.
+
+To install React with Yarn, run:
+
+```bash
+yarn add react@^15.6.0 react-dom@^15.6.0
+```
+
+To install React with npm, run:
+
+```bash
+npm install --save react@^15.6.0 react-dom@^15.6.0
+```
+
+We recommend using a bundler like [webpack](https://webpack.js.org/) or [Browserify](http://browserify.org/) so you can write modular code and bundle it together into small packages to optimize load time.
+
+Remember that by default, React runs extra checks and provides helpful warnings in development mode. When deploying your app, make sure to [compile it in production mode](/react/docs/installation.html#development-and-production-versions).
+
+In case you don't use a bundler, we also provide pre-built bundles in the npm packages which you can [include as script tags](/react/docs/installation.html#using-a-cdn) on your page:
+
+* **React**
+  Dev build with warnings: [react/dist/react.js](https://unpkg.com/react@15.6.0/dist/react.js)
+  Minified build for production: [react/dist/react.min.js](https://unpkg.com/react@15.6.0/dist/react.min.js)
+* **React with Add-Ons**
+  Dev build with warnings: [react/dist/react-with-addons.js](https://unpkg.com/react@15.6.0/dist/react-with-addons.js)
+  Minified build for production: [react/dist/react-with-addons.min.js](https://unpkg.com/react@15.5.0/dist/react-with-addons.min.js)
+* **React DOM** (include React in the page before React DOM)
+  Dev build with warnings: [react-dom/dist/react-dom.js](https://unpkg.com/react-dom@15.6.0/dist/react-dom.js)
+  Minified build for production: [react-dom/dist/react-dom.min.js](https://unpkg.com/react-dom@15.6.0/dist/react-dom.min.js)
+* **React DOM Server** (include React in the page before React DOM Server)
+  Dev build with warnings: [react-dom/dist/react-dom-server.js](https://unpkg.com/react-dom@15.6.0/dist/react-dom-server.js)
+  Minified build for production: [react-dom/dist/react-dom-server.min.js](https://unpkg.com/react-dom@15.6.0/dist/react-dom-server.min.js)
+
+We've also published version `15.6.0` of `react` and `react-dom` on npm, and the `react` package on bower.
+
+---
+
+## Changelog
+
+## 15.6.0 (June 13, 2017)
+
+### React
+
+* Fix bug where inputs mutated value on type conversion. ([@nhunzaker](https://github.com/mhunzaker) in [#9806](https://github.com/facebook/react/pull/9806))
+* Added a deprecation warning for `React.createClass`. Points users to create-react-class instead. ([@flarnie](https://github.com/flarnie) in [#9771](https://github.com/facebook/react/pull/9771))
+* Downgrade deprecation warnings to use `console.warn` instead of `console.error`. ([@flarnie](https://github.com/flarnie) in [#9753](https://github.com/facebook/react/pull/9753))
+* Fix issue where environment variable was not being transformed by browserify. ([@mridgway](https://github.com/mridgway) in [#9642](https://github.com/facebook/react/pull/9642))
+* Add deprecation warnings and separate module for React.DOM factory helpers. ([@nhunzaker](https://github.com/nhunzaker) in [#8356](https://github.com/facebook/react/pull/8356))
+* Fix bug where controlled number input mistakenly allowed period.  ([@nhunzaker](https://github.com/nhunzaker) in [#9584](https://github.com/facebook/react/pull/9584))
+* Remove warning for CSS variables. ([@aweary](https://github.com/aweary) in [#9302](https://github.com/facebook/react/pull/9302))
+* Fix bug where performance entries were being cleared.  ([@chrisui](https://github.com/chrisui) in [#9451](https://github.com/facebook/react/pull/9451))
+* Stop adding 'px' to numbers passed for unitless CSS Grid attributes.  ([@ericsakmar](https://github.com/ericsakmar) in [#9185](https://github.com/facebook/react/pull/9185))
+* Warn for deprecation of `React.createMixin` helper, which was never used. ([@aweary](https://github.com/aweary) in [#8853](https://github.com/facebook/react/pull/8853))
+
+### React DOM
+
+* Fix issues with 'onChange' not firing properly for some inputs. ([@jquense](https://github.com/jquense) in [#8575](https://github.com/facebook/react/pull/8575))
+
+### React Addons
+
+* Remove PropTypes dependency from ReactLink.  ([@gaearon](https://github.com/gaearon) in [#9766](https://github.com/facebook/react/pull/9766))