Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
There's a TODO in `usage.rst` to link to this `installation` document. Remember to do that.
`/var/build/user_builds/flocker/checkouts/0.0.4/docs/usage.rst:: WARNING: unusable reference target found: TODO:linktoourinstallationdocumentation`