* Adopt keep a changelog
Ref: https://keepachangelog.com/en/1.0.0/
* Add changelog to release and contributing docs
* Use H2 for automatically added Download section
The individual changelog subsections use H3 and the changelog section
uses H2, so the Download section should use the same heading
* Add already present changes to changelog
* Mention changelog issue references in CONTRIBUTING.md
Patrick Ziegler