add pypi tag

Author: jiahuanluo

README.md

@@ -3,6 +3,7 @@
 [![GitHub license](http://dmlc.github.io/img/apache2.svg)](./LICENSE)
 [![Documentation Status](https://readthedocs.org/projects/thundersvm/badge/?version=latest)](https://thundersvm.readthedocs.org)
 [![GitHub issues](https://img.shields.io/github/issues/Xtra-Computing/thundersvm.svg)](https://github.com/zeyiwen/thundersvm/issues)
+[![PyPI version](https://badge.fury.io/py/thundersvm.svg)](https://badge.fury.io/py/thundersvm)
 
 <div align="center">
 <img src="https://github.com/zeyiwen/thundersvm/raw/master/docs/_static/logo.png" width="240" height="220" align=left/>