Add release notes for Djblets 2.0.
Review Request #11274 — Created Nov. 12, 2020 and discarded
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.
The new contributors have also been adedd to the AUTHORS file.
Built the docs. Checked for broken links and spelling errors.