Add note to 'unreleased' CHANGELOG about deprecating Bower (#11262)

* Add note to 'unreleased' CHANGELOG about deprecating Bower

**what is the change?:**
We will no longer release new versions of React to Bower, and we should
announce that as part of our CHANGELOG.

**why make this change?:**
We decided on this as a team.

**test plan:**
Visual inspection and spell check. :)

**issue:**
Just follow-up for https://github.com/facebook/react/pull/11223

* Improve messaging/formatting

* Move bower deprecation notice to top of changelog