Add release notes for Review Board 3.0.4.
Review Request #9813 — Created March 19, 2018 and submitted — Latest diff uploaded
This covers the new Gerrit and GitLab API v4 additions, self-signed SSL
certificate support, diff viewer filtering, new template hook points,
and other fixes and features added in 3.0.4.
Built the docs and checked them for build errors and typos.