readme: add license badge

README.md

 # Universal Robot
 
 [![Build Status](http://build.ros.org/job/Kdev__universal_robot__ubuntu_xenial_amd64/badge/icon)](http://build.ros.org/job/Kdev__universal_robot__ubuntu_xenial_amd64)
+[![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
 
 [![support level: community](https://img.shields.io/badge/support%20level-community-lightgray.png)](http://rosindustrial.org/news/2016/10/7/better-supporting-a-growing-ros-industrial-software-platform)