Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
And if it is necessary, how do you force a build on a tag? I tried http://build.clusterhq.com/boxes-flocker?branch=refs/tags/0.0.2pre1 but it failed with `fatal: Remote branch refs/tags/0.0.2pre1 not found in upstream origin`