Adding README.md : a quick explanation of changes done in order to make VRPN...

Adding README.md : a quick explanation of changes done in order to make VRPN compatible with our Optitrack-ROS setup

README.md

+This is the vrpn_client_ros package that we got from here : https://github.com/ros-drivers/vrpn_client_ros/tree/indigo-devel
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+We basically modified it to be compatible with our coordinate system transformation while we are data streaming from Optitrack to ROS
+As Optitrack has y up coordinate system and ROS is a basic z up system. 
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