Data processing notes: Add for SSH access

To provide data processing information for SSH access without
annoying users too much, this adds a pointer to more extensive
information to the banner displayed after login.

The referenced files then contain similar information to what
is shown in the WebUI.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>