Clarified the requirement for using my fork of the universal_robot re…

…po (UniversalRobots#181)

README.md

@@ -103,7 +103,7 @@ $ mkdir -p catkin_ws/src && cd catkin_ws
 # clone the driver
 $ git clone https://github.com/UniversalRobots/Universal_Robots_ROS_Driver.git src/Universal_Robots_ROS_Driver
 
-# clone fork of the description to use the calibration feature
+# clone fork of the description. This is currently necessary, until the changes are merged upstream.
 $ git clone -b calibration_devel https://github.com/fmauch/universal_robot.git src/fmauch_universal_robot
 
 # install dependencies