Add a changelog file to keep track and document development

[mmattel]

I propose that a changelog file is created to summarize normal and breaking changes.

This would ease keeping track and documents development.

Could be started with new pending PR's: #320, #321, #323, #324

@wkloucek @d7oc @ainmosni @butonic @kobergj

[wkloucek]

I currently have no resources for this.

Needs at least:

• CI that enforces changelog
• Release process with changelog
• adoption of PR checklist