Adding external reference

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@63229 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_misdn.c

@@ -21,8 +21,11 @@
  * \file
  *
  * \brief the chan_misdn channel driver for Asterisk
+ *
  * \author Christian Richter <crich@beronet.com>
  *
+ * \extref MISDN http://www.misdn.org/
+ *
  * \ingroup channel_drivers
  */