README: cover uninstalling Git LFS
README: cover uninstalling Git LFS

Sometimes people decide that Git LFS isn't right for them.  Let's tell
people how to stop using it if they like, since that's a frequent source
of questions.  We also now have a FAQ entry for this, so hopefully
people will be able to find this information more easily.

github/git-lfs,github/git-lfs,github/git-lfs