dillon1000/react

Add 'unreleased' incremental change log for v15.6.0 (#9544)

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

**test plan:**
Visual inspection

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

* Minor tweaks to v15.6.0 changelog annotation

**what is the change?:**
- added missing `#` for commit hashes
- added minor details to two annotations

Thanks to @gaearon for the code review comments.

**why make this change?:**
Consistency and clarity.

**test plan:**
Visual inspection

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

* Remove hashes from commit numbers

**what is the change?:**
We had added hashes to some commit numbers, but ideally only do that
for PRs.

**why make this change?:**
Consistency - this is how github displays those types of links too. PRs
get a '#', and commits don't.

**test plan:**
Visual inspection

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