message: Add description for fanout example
diff: // except according to those terms.
  /// This example shows simple packet_fanout processing under linux.
  +/// PACKET_FANOUT in linux allows to offload packet processing to multiple threads.
  +/// See [man 7 packet](http://man7.org/linux/man-pages/man7/packet.7.html) for more details.
  extern crate pnet;
  extern crate pnet_datalink;
  