readme: add codecov

Author: mcuadros

README.md

@@ -1,4 +1,4 @@
-# go-billy [![GoDoc](https://godoc.org/gopkg.in/src-d/go-billy.v2?status.svg)](https://godoc.org/gopkg.in/src-d/go-billy.v2) [![Build Status](https://travis-ci.org/src-d/go-billy.svg)](https://travis-ci.org/src-d/go-billy) [![Build status](https://ci.appveyor.com/api/projects/status/vx2qn6vlakbi724t?svg=true)](https://ci.appveyor.com/project/mcuadros/go-billy) [![codebeat badge](https://codebeat.co/badges/03bdec03-b477-4472-bbe3-b552e3799174)](https://codebeat.co/projects/github-com-src-d-go-billy)
+# go-billy [![GoDoc](https://godoc.org/gopkg.in/src-d/go-billy.v2?status.svg)](https://godoc.org/gopkg.in/src-d/go-billy.v2) [![Build Status](https://travis-ci.org/src-d/go-billy.svg)](https://travis-ci.org/src-d/go-billy) [![Build status](https://ci.appveyor.com/api/projects/status/vx2qn6vlakbi724t?svg=true)](https://ci.appveyor.com/project/mcuadros/go-billy) [![codecov](https://codecov.io/gh/src-d/go-billy/branch/master/graph/badge.svg)](https://codecov.io/gh/src-d/go-billy) [![codebeat badge](https://codebeat.co/badges/03bdec03-b477-4472-bbe3-b552e3799174)](https://codebeat.co/projects/github-com-src-d-go-billy)
 
 An interface to abstract several storages.