Release script doesn't support patching

[rsheeter]

On occasion we need to release a patched version of an existing version. The obvious path is to rig a branch in the state you want and then release ... but the release script discourages this.

Document the process to make patch releases.

Bear in mind the script and process needs to span all our repos, not only fontations. This could be as simple as copying changes to fontc and sleipnir at time of writing.