Add coverage badge [skip ci]

README.md

@@ -4,6 +4,7 @@ Asynchronous, non-blocking [SQLite3](http://sqlite.org/) bindings for [Node.js](
 
 [![Build Status](https://travis-ci.org/mapbox/node-sqlite3.svg?branch=master)](https://travis-ci.org/mapbox/node-sqlite3)
 [![Build status](https://ci.appveyor.com/api/projects/status/gvm7ul0hpmdawqom)](https://ci.appveyor.com/project/Mapbox/node-sqlite3)
+[![Coverage Status](https://coveralls.io/repos/mapbox/node-sqlite3/badge.svg?branch=master&service=github)](https://coveralls.io/github/mapbox/node-sqlite3?branch=master)
 [![Dependencies](https://david-dm.org/mapbox/node-sqlite3.svg)](https://david-dm.org/mapbox/node-sqlite3)