**what is the change?:**
Adding a `CHANGELOG.md`

**why make this change?:**
Since this package is out of sync from React, and is more of a
stand-alone module now, we should put records of the changes in it's
own change log.

**test plan:**
visual inspection

**issue:**
facebook/react#9443

**what is the change?:**
- reformat using '[@author] in [#PR/commit]'
- Make annotations less technical and more correct

**why make this change?:**
To improve readability and consistency.

**test plan:**
Visual inspection

**issue:**
facebook/react#9443

**what is the change?:**
- Add deprecation warning to all versions pre 15.5.7.
- Move 'React package inclusion fix' annotation from 15.5.3 to 15.5.2
- Improved wording of 'Reduce size of UMD build' in 15.5.4
- Add note about 'Remove the accidentally included typechecking code...'
  in 15.5.4 (what commit did this?)
- Move 'Remove accidental uncompiled ES6' annotation to 15.5.1
- That leaves no changes under 15.5.3; maybe we released this in an
  effort to keep this in sync with the main React package?

Follow up commit adds annotations for 15.5.4-15.5.8.

**why make this change?:**
To be more accurate.

**test plan:**
Visual inspection.

**issue:**
facebook/react#9443

**what is the change?:**
See title.

**why make this change?:**
To provide information about the version history, and to get people
updated to the latest version of this package.

**test plan:**
Visual inspection.

**issue:**
facebook/react#9443

**what is the change?:**
Another correction to the change log, based on helpful comments from
@gaeaeron.

**why make this change?:**
Accuracy

**test plan:**
Visual inspection

**issue:**
facebook/react#9443

**what is the change?:**
Adding two missing parens and a note about the initial release version.

**why make this change?:**
Clarity/consistency

**test plan:**
Visual inspection

**issue:**
facebook/react#9443