Add PR facebook#9642 to 15.6 change log

Just keeping this fresh.

Issue:

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### React
 
+* 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))
 * Fix bug where performance entries were being cleared.  ([@chrisui](https://github.com/chrisui) in [#9451](https://github.com/facebook/react/pull/9451))