[AIRFLOW-208] Add badge to show supported Python versions (apache#3839)

Author: kaxil

README.md

@@ -5,6 +5,7 @@
 [![Coverage Status](https://img.shields.io/codecov/c/github/apache/incubator-airflow/master.svg)](https://codecov.io/github/apache/incubator-airflow?branch=master)
 [![Documentation Status](https://readthedocs.org/projects/airflow/badge/?version=latest)](https://airflow.readthedocs.io/en/latest/?badge=latest)
 [![License](http://img.shields.io/:license-Apache%202-blue.svg)](http://www.apache.org/licenses/LICENSE-2.0.txt)
+[![PyPI - Python Version](https://img.shields.io/pypi/pyversions/apache-airflow.svg)](https://pypi.org/project/apache-airflow/)
 [![Join the chat at https://gitter.im/apache/incubator-airflow](https://badges.gitter.im/apache/incubator-airflow.svg)](https://gitter.im/apache/incubator-airflow?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 _NOTE: The transition from 1.8.0 (or before) to 1.8.1 (or after) requires uninstalling Airflow before installing the new version. The package name was changed from `airflow` to `apache-airflow` as of version 1.8.1._