document a systemd deployment

This hasn't actually been tested but, really, "how hard can this be?"

Once concern is that systemd might not take care of locking and that a job might take more than one hour and run in parallel. We're pretty good at avoiding race conditions (by moving stuff to dangerzone/processing, mostly) but we definitely should avoid two processors in parallel. So we should make sure that only one instance can be started by systemd, which is unclear to me right now.

Closes: #1 (closed)