Add pypi badge to README.md

Author: jhgg

README.md

@@ -2,6 +2,7 @@
 
 Core GraphQL Library for Python
 
+[![PyPI version](https://badge.fury.io/py/graphql-core.svg)](https://badge.fury.io/py/graphql-core)
 [![Build Status](https://travis-ci.org/graphql-python/graphql-core.svg?branch=master)](https://travis-ci.org/graphql-python/graphql-core)
 [![Coverage Status](https://coveralls.io/repos/graphql-python/graphql-core/badge.svg?branch=master&service=github)](https://coveralls.io/github/graphql-python/graphql-core?branch=master)
 [![Public Slack Discussion](https://graphql-slack.herokuapp.com/badge.svg)](https://graphql-slack.herokuapp.com/)