Merge pull request #5989 from thomasrockhu/codecov-badge

Add Codecov badge to README

README.md

@@ -3,8 +3,7 @@
 [![Google Group](https://img.shields.io/badge/-Google%20Group-lightgrey.svg)](https://groups.google.com/forum/#!forum/jupyter)
 [![Build Status](https://travis-ci.org/jupyter/notebook.svg?branch=master)](https://travis-ci.org/jupyter/notebook)
 [![Documentation Status](https://readthedocs.org/projects/jupyter-notebook/badge/?version=latest)](https://jupyter-notebook.readthedocs.io/en/latest/?badge=latest)
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+[![codecov](https://codecov.io/gh/jupyter/notebook/branch/master/graph/badge.svg)](https://codecov.io/gh/jupyter/notebook)
 
 The Jupyter notebook is a web-based notebook environment for interactive
 computing.