Add release notes for Djblets 2.0.
Review Request #11148 — Created Aug. 20, 2020 and updated
This is a very large release, with many new features and architectural
improvements. The big highlights are the Django 1.11 and Python 3
support, which involved a lot of changes all throughout the codebase.
Built the docs. Checked for build errors, link errors, and spelling errors.