Try and document how subdirs works with git repositories.
Try and document how subdirs works with git repositories.

This tries to cover my confusion leading to #3162 and
https://stackoverflow.com/questions/43752580/git-location-with-subdir-but-not-root-directory

MichielDerhaeg/stack,anton-dessiatov/stack,anton-dessiatov/stack,MichielDerhaeg/stack,juhp/stack,juhp/stack