[Fedora-packaging] Revised systemd Guidelines

Many thanks to Lennart for writing the initial text for the systemd guidelines. I've taken his draft and reworked it a bit so that it matches the flow and style of the existing guidelines: https://fedoraproject.org/wiki/TomCallaway/Systemd_Revised_Draft Keep in mind that %{_unitdir} is undefined at the moment, we would add it to redhat-rpm-config before this draft becomes a guideline. Please look it over and give any feedback. Thanks, ~tom == Fedora Project