Document process for release

[lossyrob]

Document the process for version tagging a release and generating the appropriate migration scripts.

This is follow-up work from #18, which introduced a release process that is based on GitHub releases, so that releases are automated based on a new tag/GitHub release.

The Makefile processing currently creates a script that inserts the version in the migration table. This issue would create a similar workflow that is done as a manual step prior to releasing.