Add release notes for Review Board 3.0.9.

Review Request #10115 — Created Aug. 6, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
df2f40e...

Reviewers

This is a bug fix release that addresses loading/display issues in
custom text fields for extensions, visual issues with move detection
flags, Subversion compatibility, WebHook data type validation, Last
Updated resource fixes, and more.

Built the docs. Checked for bad links, build errors, and spelling
errors.