Update README.md

Author: AtsushiSakai

README.md

@@ -4,6 +4,7 @@
 [![Build Status](https://travis-ci.org/AtsushiSakai/PythonRobotics.svg?branch=master)](https://travis-ci.org/AtsushiSakai/PythonRobotics)
 [![Documentation Status](https://readthedocs.org/projects/pythonrobotics/badge/?version=latest)](https://pythonrobotics.readthedocs.io/en/latest/?badge=latest)
 [![Build status](https://ci.appveyor.com/api/projects/status/sb279kxuv1be391g?svg=true)](https://ci.appveyor.com/project/AtsushiSakai/pythonrobotics)
+[![Coverage Status](https://coveralls.io/repos/github/AtsushiSakai/PythonRobotics/badge.svg?branch=master)](https://coveralls.io/github/AtsushiSakai/PythonRobotics?branch=master)
 [![Say Thanks!](https://img.shields.io/badge/Say%20Thanks-!-1EAEDB.svg)](https://saythanks.io/to/AtsushiSakai)
 
 Python codes for robotics algorithm.