Add release notes for Review Board 2.0.16.

Review Request #7350 — Created May 28, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
5bf45c1...

Reviewers

This covers some of the new features (GitLab post-commit support,
Mercurial-generated Git diffs), the performance improvements, and all
the bug fixes made since 2.0.15.

Built the docs and read through them.