Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
Move the `flocker-node` section to the bottom, so `flocker-cli` is first. Most people shouldn't be manually installing `flocker-node` at this point, so no point in having them have to do the work to skip to part they should care about.