Add 'Unreleased' section to CHANGELOG

[flarnie]

Let's make releases a bit faster/easier by keeping the CHANGELOG updated incrementally.

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.

[flarnie]

Getting this rolling will be tricky initially, but I think it's important in order to help us keep track of which features are present on master vs. the 15.* branches. Also related to #9443

[flarnie]

Relevant to this issue - (edit:) it seems like there is a big divergence between the v15.6 branch and master. If there really are 2k+ commits in v16.0 that are not present on the v15.6 branch, then writing the changelog is going to be a serious chunk of work.