Use a changelog instead of current release notes

A changelog is more commonly used in software projects and supported/suggested by the gitlab interfaces.

We should replace the current releases notes with a changelog, possibly following the guidelines at http://keepachangelog.com/en/1.0.0/

The main issue is that the log should be updated on the master branch, which conflicts with the current policy of allowing changes to master only via approved merge requests. If we all agree we could decide to bypass the policy for the changelog.