libcgroup: Added README_systemd file.

libcgroup: Added README_systemd file.

New documentation file is added. It should describe how systemd and
libcgroup should work together and how to configure a system to have
control groups created by libcgroup's cgconfig service.

I'm going to install this README into /usr/share/doc/libcgroup-*/.

I know I am not Shakespeare and also English is not my mother language, feel
free to comment also the style and grammar. Missing/wrong articles is my
speciality :).

Changelog:
  - updated with review remarks from Martin Prpic and Kevin Constantine

Signed-off-by: Jan Safranek <jsafrane@redhat.com>