Packaging: Add howto enable a systemd service

[TraceyC77]

We now have the ability to have a package systemd service enabled at install time (default is disabled)
Add docs to packaging on

• Why you would want to do this
• Packages where this should not be done for ex security reasons (ex: sshd)
• How to enable the systemd service in a package

[sheepman4267]

And we need to most definitely document the heck out of the fact that preset files are:
1: Read in the order of priority (like everything else)
2: Applied in reverse order for some unfathomable reason, so 99-whatever.preset is overridden by any preset file which was read before.

What even?

Whatever....