Documentation: User Units

[corporealfunk]

Documentation Update:

If you are using a "user" service, and you want the units to start on boot, you will need to the following:

1. As root or sudo: loginctl enable-linger USERNAME
   This ensures the user specified by USERNAME can run services outside of a login session.
2. In your repo, your Unit must specify the following WantedBy target (the multi-user.target will not work in a user unit):

    [Install]
    WantedBy = default.target

[HLFH]

@corporealfunk I have added the documentation, the code fixes from @dapi (+ completed the rspec tests) in this fork: https://github.com/HLFH/capistrano-systemd-ng.