DOC: add Python versions badge

Author: davidesarra

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/davidesarra/jupyter_spaces.svg?branch=master)](https://travis-ci.org/davidesarra/jupyter_spaces)
 [![codecov](https://codecov.io/gh/davidesarra/jupyter_spaces/branch/master/graph/badge.svg)](https://codecov.io/gh/davidesarra/jupyter_spaces)
+[![Supported Python Versions](https://img.shields.io/pypi/pyversions/jupyter_spaces.svg)](https://pypi.org/project/jupyter_spaces/)
 
 Jupyter Spaces is an IPython extension for creating parallel namespaces
 availabe within the user namespace. It is designed to be used via IPython