add coverage stats

Author: Bhupesh-V

README.md

@@ -4,11 +4,11 @@
 
 ![PyPI](https://img.shields.io/pypi/v/coderunner?color=blue)
 [![Build Status](https://travis-ci.org/codeclassroom/CodeRunner.svg?branch=master)](https://travis-ci.org/codeclassroom/CodeRunner)
+[![codecov](https://codecov.io/gh/codeclassroom/CodeRunner/branch/master/graph/badge.svg)](https://codecov.io/gh/codeclassroom/CodeRunner)
 ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/coderunner)
 [![Documentation Status](https://readthedocs.org/projects/coderunner/badge/?version=latest)](https://coderunner.readthedocs.io/en/latest/?badge=latest)
 ![PyPI - Downloads](https://img.shields.io/pypi/dm/coderunner?color=blue)
 [![GitHub license](https://img.shields.io/github/license/codeclassroom/CodeRunner)](https://github.com/codeclassroom/CodeRunner/blob/master/LICENSE)
-[![GitHub issues](https://img.shields.io/github/issues/codeclassroom/CodeRunner?color=blueviolet)](https://github.com/codeclassroom/CodeRunner/issues)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-orange.svg)](http://makeapullrequest.com)