Add topology in sample server configuration file

Add topology in sample server configuration file

This is an OpenPGP/MIME signed message (RFC 4880 and 3156)
On modern systems, topology subnet should always be set, but it's
missing in the configuration file.
Add it with a short explanation.

Signed-off-by: Philipp Hagemeister <phihag@phihag.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <53BF9998.5020906@phihag.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8878
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c277757fcf7fb4c2713db154439f937d48cfae61)