docs(readme): fix coverage badge

Author: dignifiedquire

README.md

@@ -3,7 +3,7 @@
 [![](https://img.shields.io/badge/made%20by-Protocol%20Labs-blue.svg?style=flat-square)](http://ipn.io)
 [![](https://img.shields.io/badge/freenode-%23ipfs-blue.svg?style=flat-square)](http://webchat.freenode.net/?channels=%23ipfs)
 [![Build Status](https://travis-ci.org/libp2p/js-libp2p-tcp.svg?style=flat-square)](https://travis-ci.org/libp2p/js-libp2p-tcp)
-![](https://img.shields.io/badge/coverage-%3F-yellow.svg?style=flat-square)
+[![Coverage Status](https://coveralls.io/repos/github/libp2p/js-libp2p-tcp/badge.svg?branch=master)](https://coveralls.io/github/libp2p/js-libp2p-tcp?branch=master)
 [![Dependency Status](https://david-dm.org/libp2p/js-libp2p-tcp.svg?style=flat-square)](https://david-dm.org/libp2p/js-libp2p-tcp)
 [![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat-square)](https://github.com/feross/standard)