add coverage status

README.md

@@ -1,6 +1,6 @@
 <a href="#"><img alt="rex" src="https://raw.githubusercontent.com/go-rex/rex/assets/images/rex.png" width="160px" height="64px"></a>
 ===
-[![Build Status](https://travis-ci.org/goanywhere/rex.svg?branch=master)](https://travis-ci.org/goanywhere/rex) [![GoDoc](https://godoc.org/github.com/goanywhere/rex?status.svg)](http://godoc.org/github.com/goanywhere/rex)
+[![Build Status](https://travis-ci.org/goanywhere/rex.svg?branch=master)](https://travis-ci.org/goanywhere/rex) [![Coverage](http://gocover.io/_badge/github.com/goanywhere/rex?0)](http://gocover.io/github.com/goanywhere/rex) [![GoDoc](https://godoc.org/github.com/goanywhere/rex?status.svg)](http://godoc.org/github.com/goanywhere/rex)
 
 Rex is a library for modular web development in [Go](http://golang.org/), designed to work directly with net/http.