Document release process

Releasing is now easy!

Follow the docs in CONTRIBUTING.md to release a new version.

1. Create a `release-x.y.z` branch off of master.
1. Review the merge commits with `git log --merges ${last_tag}..HEAD` and update `CHANGELOG.md`.
1. Bump the carbonate version in `carbonate/__init__.py`.
1. Push the branch to GitHub and create a PR for discussion.
1. Merge the PR.
1. Run `script/release`.