Add coverage status

README.md

-[![GoDoc](https://godoc.org/github.com/gwenn/gosqlite?status.svg)](https://godoc.org/github.com/gwenn/gosqlite) [![Build Status][1]][2] [![Go Report Card](https://goreportcard.com/badge/github.com/gwenn/gosqlite)](https://goreportcard.com/report/github.com/gwenn/gosqlite)
+[![GoDoc](https://godoc.org/github.com/gwenn/gosqlite?status.svg)](https://godoc.org/github.com/gwenn/gosqlite) [![Build Status][1]][2] [![Go Report Card](https://goreportcard.com/badge/github.com/gwenn/gosqlite)](https://goreportcard.com/report/github.com/gwenn/gosqlite) [![Coverage Status](https://coveralls.io/repos/github/gwenn/gosqlite/badge.svg?branch=master)](https://coveralls.io/github/gwenn/gosqlite?branch=master)
 
 [1]: https://secure.travis-ci.org/gwenn/gosqlite.png
 [2]: http://www.travis-ci.org/gwenn/gosqlite