Add release notes for Review Board 2.0.18.

Review Request #7464 — Created June 26, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
407d4a9...

Reviewers

This covers the new extensibility improvements in the comment dialog,
Perforce/Subversion fixes, API bullet-proofing, and everything else
fixed since 2.0.17.

Built the docs and looked through them.