pkg: systemd: add initial MAINTAINERS

I volunteered for pkg/systemd MAINTAINER and there were no objections
during the #docker-dev meeting. For context I wrote most of the stuff in
here and wrote the dependent calls in api.go. Plus, I actively test the
code via CoreOS.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)