Add 'unreleased' incremental change log for v15.6.0

**what is the change?:**
Adding a section to the change log where we start accumulating
annotations for React v15.6.0.

**why make this change?:**
- Saves us the trouble of writing the change log entry all at once when
  we do the release.
- Adds transparency about what is in the upcoming release, for those who
  aren't following #9398

**test plan:**
Visual inspection

**issue:**
#9398

CHANGELOG.md

@@ -1,3 +1,15 @@
+## 15.6.0 [Unreleased]
+<details>
+ <summary>Changed we plan to include in the 15.6.0 release.</summary>
+
+### React
+
+* 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 CSS Grid attributes.  ([@ericsakmar](https://github.com/ericsakmar) in [9185](https://github.com/facebook/react/pull/9185))
+* Deprecate `React.createMixin`. ([@aweary](https://github.com/aweary) in [#8853](https://github.com/facebook/react/pull/8853))
+
+</details>
+
 ## 15.5.0 (April 7, 2017)
 
 ### React