This could really use extra code review attention since the history of
these changes was a bit convoluted to follow.

After talking to @bvaughn and @acdlite, we thought it might make sense
to put the 'add-ons' changes in a separate change log. The other option,
of including them in the main React change log, seemed the more
confusing of the two.

Also this commit is related to and somewhat blocked by
facebook/prop-types#40

**what is the change?:**
Adding the change log for recent patch versions of React.

**why make this change?:**
We missed this step in the flurry of releasing patches, and it's useful
for folks who want info about what version to use.

**test plan:**
Visual inspection of the change log.

**issue:**
facebook#9443

**what is the change?:**
- Use the '[@author] in [#PR/commit]' format for annotations
- Make annotations less technical, more clear
- Move 'React Addons' updates into main changelog
- Remove separate 'React Addons' changelog

**why make this change?:**
These changes each make things more clear and accurate.

**test plan:**
Visual inspection

**issue:**

**what is the change?:**
- Put backticks around package names
- Reformat link to to commit in '([@user](...) in [#NNNN](...))' format
- Remove newlines after subheaders; in the past we sometimes have
  included a newline after the subheader, but most recently it looks
  like we do not.
- Add some missing punctuation.

**why make this change?:**
Consistency and aesthetics

**test plan:**
Visual inspection

**issue:**
facebook#9443

**what is the change?:**
Adding deprecation notice to some recent React versions.

**why make this change?:**
These versions of React use a version of `prop-types` that had a
critical bug. We updated the dependency in React 15.5.4, and hopefully
people will see this notice and update.

**test plan:**
Visual inspection

**issue:**
facebook#9537