Add plain-text overview of version details
One of our usage patterns is: "Download whole directories from a centrally deployed copy." This breaks the git-references, because we use worktree. To help these users to at least be able to know the exact version of the data (even if their copy is not a valid Git repo anymore), we should add a file with the output of something like:
$ git describe --tags
$ git remote -v
$ # what else?