Add comment about the importance of AsyncSpinner (#565)

* Add comment about the importance of AsyncSpinner

Author: samarth-robo

doc/move_group_interface/src/move_group_interface_tutorial.cpp

@@ -49,6 +49,10 @@ int main(int argc, char** argv)
 {
   ros::init(argc, argv, "move_group_interface_tutorial");
   ros::NodeHandle node_handle;
+
+  // ROS spinning must be running for the MoveGroupInterface to get information
+  // about the robot's state. One way to do this is to start an AsyncSpinner
+  // beforehand.
   ros::AsyncSpinner spinner(1);
   spinner.start();