Add PR #9302 to 15.6RC CHANGELOG (#9857)

Just keeping things updated.

#9398

CHANGELOG.md

@@ -10,6 +10,7 @@
 * 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))