add download stats

Author: Bhupesh-V

README.md

@@ -4,6 +4,7 @@
 [![Build Status](https://travis-ci.org/codeclassroom/PlagCheck.svg?branch=master)](https://travis-ci.org/codeclassroom/PlagCheck)
 ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/plagcheck)
 [![GitHub license](https://img.shields.io/github/license/codeclassroom/PlagCheck)](https://github.com/codeclassroom/CodeRunner/blob/master/LICENSE)
+![PyPI - Downloads](https://img.shields.io/pypi/dm/plagcheck?color=blue)
 [![GitHub issues](https://img.shields.io/github/issues/codeclassroom/PlagCheck?color=blueviolet)](https://github.com/codeclassroom/CodeRunner/issues)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-orange.svg)](http://makeapullrequest.com)