add changelog file

[vincanger]

No description provided.

[Martinsos]

Hm what would be the process around it? We could, for every "release" we do, add changes from the last one to the Changelog.
One option is also to stick to the process where each PR has to add info about itself to CHangelog. Although, you cuold say people can then just go and check the list of PRs on Github hm.