dillon1000/react

Update changelog for unreleased 16.0 changes (#10730)

* First shot at updating changelog for 16.0

**what is the change?:**
Added an 'unreleased' section to the changelog with info from https://github.com/facebook/react/issues/10294

**why make this change?:**
To get things set for the 16.0 release.

**test plan:**
Visual inspection

**issue:**
https://github.com/facebook/react/issues/8854

* Fix typos and formatting errors in changelog

* Add requestAnimationFrame and remove "New helpful warnings"

**what is the change?:**
In response to helpful code review -
- Add mention of dependency on `requestAnimationFrame` and need to
  polyfill that as well as `Map` and `Set`
- Remove "New helpful warnings" section; it was incomplete, and there
  are so many new and updated warnings that it might not be reasonable
  to cover them in the changelog.

**why make this change?:**
Accuracy

**test plan:**
Visual inspection

**issue:**
issue #8854

* Improve wording

* Improve wording and fix missing links

* Add backticks to file names & code; wording tweak

* Break "Major Changes" into "New Feature" and "Breaking Changes"

* Add server side render changes to 16.0 changelog

* Change gist link from mine to gaearons

* Add note about returning fragments

* fix misc nits

* Misc. formatting/wording fixes to changelog

**what is the change?:**
Thanks to the kind code review comments of @gaearon and @nhunzaker we
have
- removed the non-deterministic bold styling on some bullet points
- improved wording of the bullet points for portals, attribute whitelist
  changes, and server rendering changes
- Add note about error boundaries including a breaking change to error
  handling behavior.
- punctuation and capitalization fixes

**why make this change?:**
Clarity and correctness

**test plan:**
Visual inspection

**issue:**
https://github.com/facebook/react/issues/8854

* fix broken link