doc(README): add codecov badge

[skip ci]

Author: Byron

README.md

@@ -129,6 +129,7 @@ New BSD License.  See the LICENSE file.
 
 ### DEVELOPMENT STATUS
 
+[![codecov](https://codecov.io/gh/gitpython-developers/GitPython/branch/master/graph/badge.svg)](https://codecov.io/gh/gitpython-developers/GitPython)
 [![Build Status](https://travis-ci.org/gitpython-developers/GitPython.svg)](https://travis-ci.org/gitpython-developers/GitPython)
 [![Build status](https://ci.appveyor.com/api/projects/status/0f3pi3c00hajlrsd/branch/master?svg=true&passingText=windows%20OK&failingText=windows%20failed)](https://ci.appveyor.com/project/Byron/gitpython/branch/master)
 [![Code Climate](https://codeclimate.com/github/gitpython-developers/GitPython/badges/gpa.svg)](https://codeclimate.com/github/gitpython-developers/GitPython)