Add a changelog

[nbaksalyar]

Since breaking changes are inevitable, I think introducing a changelog would be a good idea.

I propose to follow the format described on keepachangelog.com. Usually, it's not a huge maintenance burden: contributors guide can be updated to require updating the changelog with each PR (especially if the change is breaking).

[rsdy]

We are currently tracking changes on the Releases page, which is reasonably easy to extract from the git logs. However, I would generally not approve of any "better" change tracking mechanism that we can't enforce through a CI check.

If you have any suggestions, then we could probably have a conversation on setting up a policy to make this easier.