Add release notes for Review Board 4.0.
Review Request #11606 — Created May 14, 2021 and submitted — Latest diff uploaded
This covers all the features, improvements, and fixes that went into
Review Board 4.0, along with installation instructions, upgrade notes,
and comaptibility changes.It's a big release with a lot to cover. The focus on the release notes
was to discuss the installation-related topics first, and to follow that
up with all the new feature changes, followed by bug fixes.
Built the docs. Checked for build errors, link errors, and spelling
errors.