Add "Changelog" section to the documentation

We should populate the "in development" section as the pull requests are being merged in so it's always up to date.

This way, users can just check this document to see what's coming in the future and what's new in master instead of needing to browse through the commits.

The same document should also serve as an official change log document (before we prepare and cut a release we should just go over it and clean it up as needed).