Add release notes for 2.0.21.

Review Request #7792 — Created Dec. 3, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x

Reviewers

This covers all the various changes that have gone into this release,
including bug fixes for post-commit review, diff caching, GitLab, SSH,
and more. It also contains the new Assembla feature, e-mail
improvements, and extension improvements.

Built the docs and checked through them. Looked fine.