message: mavsdk_server: note about automatic port selection
diff: @@ -59,7 +59,8 @@ void usage(const char* bin_name)
  << '\n'
  << "Options:" << '\n'
  << " -h | --help : show this help" << '\n'
  - << " -p : set the port on which to run the gRPC server\n"
  + << " -p : set the port on which to run the gRPC server,\n"
  + << " set to 0 to choose a free port automatically\n"
  << " (default is " << default_mavsdk_server_port << ")\n";
  }
  