Document release process.

Fixes #292.

This is mostly an initial sketch for discussion.

This also at least partially addresses #86, #87, #126, #277.
The steps above imply (but don't explain), that releases are made from a branch that looks like `release/flocker-X.Y`, for releases `X.Y.Z`. This is similar to how twisted does it, but was motivated more to make detecting them on buildbot (https://github.com/ClusterHQ/build.clusterhq.com/pull/5) easier.

With the hybridcluster buildbot, we would sometimes have non-release branches that had a `hybridcluster-` prefix, which got incorrectly treated as release branches.