doc(README): add appveyor badge

[skip ci]

README.md

@@ -125,6 +125,7 @@ New BSD License.  See the LICENSE file.
 ### DEVELOPMENT STATUS
 
 [![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)
 [![Documentation Status](https://readthedocs.org/projects/gitpython/badge/?version=stable)](https://readthedocs.org/projects/gitpython/?badge=stable)
 [![Stories in Ready](https://badge.waffle.io/gitpython-developers/GitPython.png?label=ready&title=Ready)](https://waffle.io/gitpython-developers/GitPython)