Add release notes for Review Board 3.0.18.
Review Request #1996 — Created Sept. 7, 2020 and updated
This release has some compatibility fixes for various hosting services and SCMs, bug fixes for the API and many other parts of the product, enhancements to `condensediffs`, and performance improvements for SSH. Testing Done: Built the release notes. Checked for build errors, link errors, and spelling errors. Reviewed at https://reviews.reviewboard.org/r/11056/