**what is the change?:**
- Add 'Unreleased' section to `CHANGELOG`
- Add step to 'contributing' about updating the `CHANGELOG`

**why make this change?:**

Whoever does the release of a new React version must manually read
through the recent git history and write the CHANGELOG update.

If we incrementally add to the CHANGELOG under the 'Unreleased' header,
as we make PRs, then to release a new version will just be updating
'Unreleased' to be the new version number and adding a blank section for
'Unreleased'.

See http://keepachangelog.com/en/0.3.0/ for more info about this idea.

**test plan:**
Visual verification of the change, and also I built the website locally
and inspected the change.

**issue:**
facebook#9527

**what is the change?:**
Fix the numbers in a list

**why make this change?:**
They were incorrect due to an oversight in the previous commit.

**test plan:**
Look at the docs.

**issue:**
facebook#9527

**what is the change?:**
see title

**why make this change?:**
This will make the change log easier to read for folks who are just
looking for info about the latest stable release.

**test plan:**
Visual inspection.

**issue:**