Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
This should in future be some sort of python script, rather than trying to
get shell expansion right. I think for 0.1.0 (which we probably won't have
a prerelease for), the current expansion is probably enough. (The point of
it is to remove the micro-version in the name of the branch).