Automate generation of release notes

[raoulmillais]

git log --merges --format=%b $(git describe --tags --abbrev=0)..HEAD will take the body and author of all the merge commits since the last tag and format them nicely.

We should :

• add a Make target to do this
• append to a RELEASE_NOTES.md file in the root of the repo and commit it as part of the release target
• add the notes to the release in github as part of the release process

[github-actions]

Dying issue, will be closed soon