Add Windows build status badge.

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/jelmer/dulwich.png?branch=master)](https://travis-ci.org/jelmer/dulwich)
+[![Windows Build status](https://ci.appveyor.com/api/projects/status/cnothr6pxprfx2lf/branch/master?svg=true)](https://ci.appveyor.com/project/jelmer/dulwich-njb6g/branch/master)
 
 This is the Dulwich project.