Add auto-tagging for merges that update the version

[pganssle]

Once #5 is done, we should have a simple mechanism for detecting when the version has been updated (either by the update script or by the version bumping script), and if #4 is release automation based on tags, it would be good for us to add some mechanism for automatically tagging the repository whenever the version is bumped.

Another option is to try to go the other way: trigger the release script based on updates to the VERSION file and generate and push a new tag as part of the release script.