Add debug section in docs

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>

docs/index.rst

@@ -175,6 +175,16 @@ In case you use ferm, this file should be included in `/etc/ferm/ferm.conf`.
 Otherwise an `rc.local` script can be used to issue those rules upon boot.
 
 
+debug
+-----
+
+A useful way to see the clients registered in nfdhpcd runtime context one can
+send SIGUSR1 and see the list in the logfile:
+
+.. code-block:: console
+
+ # kill -SIGUSR1 $(cat /var/run/nfdhcpd/nfdhpcd.pid) && tail -n 100 /var/log/nfdhcpd/nfdhpcd.log
+
 
 | [1] https://www.wzdftpd.net/redmine/projects/nfqueue-bindings/wiki/
 | [2] https://code.grnet.gr/projects/snf-network/