Add release notes for Review Board 2.5.2.

Review Request #7791 — Created Dec. 3, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x

Reviewers

This covers the various bug fixes for image review, GitLab, webhooks,
condensediffs, e-mails, and more, along with the new helpful
instructions for getting GitHub Enterprise or TFS repositories
configured.

Built the docs and tested the links. Didn't see any errors.