docs: Auto-create CHANGES from git

[sijis]

This dynamically creates CHANGES.rst using gitchangelog.

Since this file is shared with the website, a generated output looks like below:

[sheluchin]

It would be nice to use something like https://github.com/c4milo/github-release as well to give https://github.com/errbotio/errbot/releases a bit more meaning and to include commit hashes next to each change. Maybe gitchangelog can be configured to include the commit hash? I think it would make it easier to understand each change.