add codecov badge

README.md

@@ -3,6 +3,7 @@
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/85221cd3bb6e49d7bbd6fed376a88264)](https://www.codacy.com/app/graphite-project/carbon?utm_source=github.com&utm_medium=referral&utm_content=graphite-project/carbon&utm_campaign=badger)
 [![Build Status](https://secure.travis-ci.org/graphite-project/carbon.png?branch=master)](http://travis-ci.org/graphite-project/carbon)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fgraphite-project%2Fcarbon.svg?type=shield)](https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fgraphite-project%2Fcarbon?ref=badge_shield)
+[![codecov](https://codecov.io/gh/graphite-project/carbon/branch/master/graph/badge.svg)](https://codecov.io/gh/graphite-project/carbon)
 
 ## Overview