Docs Linux Migration path for TFVC to Git

Article: https://docs.gitlab.com/ee/user/project/import/tfs.html

We recently needed to migrate a TFVC repo to git, however git-tfs was not an option, since it's Windows only. We came up with our own solution, which I've open-sourced here: https://github.com/turbo/gtfotfs

If anyone else needs to run an unattended migration on unix systems, I think it would be good to include this solution in the article.