add PyPI version badge.

README.md

@@ -2,6 +2,7 @@
 [![Build Status](https://travis-ci.org/jajupmochi/graphkit-learn.svg?branch=master)](https://travis-ci.org/jajupmochi/graphkit-learn)
 [![codecov](https://codecov.io/gh/jajupmochi/graphkit-learn/branch/master/graph/badge.svg)](https://codecov.io/gh/jajupmochi/graphkit-learn)
 [![Documentation Status](https://readthedocs.org/projects/graphkit-learn/badge/?version=master)](https://graphkit-learn.readthedocs.io/en/master/?badge=master)
+[![PyPI version](https://badge.fury.io/py/graphkit-learn.svg)](https://badge.fury.io/py/graphkit-learn)
 
 A python package for graph kernels.