[Fedora-packaging] Revised systemd Guidelines

[Tom Callaway]

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

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