Bug#820111: Info received (document how to use per-user systemd --user services)

[Patrick Schleizer]

By Debian stretch default, there are some default systemd user unit
files in `/usr/lib/systemd/user/`.

With a different path on Debian stretch
`/usr/lib/systemd/user/mytest.service`:

```
[Unit]
Description=mytest

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/true

[Install]
WantedBy=default.target
```

    systemctl --user enable mytest

Worked for me.

> `Created symlink
/home/user/.config/systemd/user/default.target.wants/mytest.service →
/usr/lib/systemd/user/mytest.service.`

After reboot, mytest.service will be active.