Update the contribution information and templates.
The contribution information and templates were growing a bit stale,
given that they haven't gotten a refresh since ~1.3.0. This commit
reworks them to be more thorough and straightfoward.
Add a CHANGELOG document that keeps track of the past and current
release notes.
Making all of our releases rely on GitHub to keep track of them is not
really a good idea, so this document serves as a record of past release
changelogs just as a pre-caution. It also allows a live preview of the
next version whenever something is changed.