systemd: add documentation and a sample mail.target

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

contrib/systemd/README.md

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+Systemd units
+=============
+
+These unit files are meant to be used in the user session. You may drop them
+into `${XDG_DATA_HOME}/systemd/user` followed by `systemctl --user
+daemon-reload` to have systemd aware of the unit files.
+
+These files are meant to be triggered either manually using `systemctl --user
+start offlineimap.service` or by enabling the timer unit using `systemctl
+--user enable offlineimap.timer`. Additionally, specific accounts may be
+triggered by using `offlineimap@myaccount.timer` or
+`offlineimap@myaccount.service`.
+
+These unit files are installed as being enabled via a `mail.target` unit which
+is intended to be a catch-all for mail-related unit files. A simple
+`mail.target` file is also provided.

contrib/systemd/mail.target

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+[Unit]
+Description=Mail Target
+
+[Install]
+WantedBy=default.target