Add a CHANGELOG

[robo9k]

Especially with automated releases upon git tags as in #5 , the releases should have a text snippet explaining the changes from the previous published version.
Those snippets should mention how users can fix their code (if possible) if the version contains breaking changes as per semver.
The snippets should not be generated from git commit messages, as those are often not helpful (e.g. "Update dependencies", "Fix links", "Add initial AppVeyor config" and such).

The same snippets should be contained in a CHANGELOG (or possibly CHANGELOG.md) file.
The bumpversion README looks very similar to what I propose here.