Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
We're suggesting `homebrew` for the tutorial, so I think we should give instructions based on that. As such, I suggest we do not close the OS X issue when merging this PR, and change this appropriately to use homebrew to resolve that issue (and if we run out of time at least we'll have something).