Update CHANGELOG for unreleased 15.6 branch (#9827)

* Update CHANGELOG for unreleased 15.6 branch

**what is the change?:**
Added entries for the latest changes.

**why make this change?:**
We are about to do an RC release and folks can look at these notes to
see what is in the planned release.

**test plan:**
Visual inspection

**issue:**
https://github.com/facebook/react/issues/9398

* Update changelog regarding `createClass` dep. warning

**what is the change?:**
We decided to strike through the previous changelog item and add it to
the more recent changelog section, since this is the release where the
deprecation warning will actually go out.

**why make this change?:**
To make things clear for everyone using 15.6

**test plan:**
Visual inspection

**issue:**
https://github.com/facebook/react/issues/9398