Consolidate all developer documentation a dedicated CONTRIBUTING.md file.
Rewrite the README.md to better describe the project and provide only info targeted at users (most technical/dev info moved to CONTRIBUTING.md).
Make the release notes file only about release notes (credits moved to README.md).
This diff mostly moves paragraphs and section between files, with only minor additions:
- link the release notes in README
- mention the functional tests in the "Running tests" section. This was previously undocumented.