Symlink vendor/MAINTAINERS over to hack/MAINTAINERS (since it depends explicitly on hack/vendor.sh)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3624 from tianon/vendor-maintainers

Symlink vendor/MAINTAINERS over to hack/MAINTAINERS