Add release notes for Review Board 3.0.6.

Review Request #9863 — Created April 9, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
4c2ab8a...

Reviewers

This is a small bug fix release that fixes compatibility issues with
GitLab, Bazaar, and Mercurial, along with Local Site URL-related issues
and links in e-mails.

Built the docs and checked through them for issues.

Checked for spelling and build errors.