add AppVeyor badge to show Windows build status

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/w23/OpenSource.png)](https://travis-ci.org/w23/OpenSource)
+[![Build Status](https://travis-ci.org/w23/OpenSource.png)](https://travis-ci.org/w23/OpenSource) [![Build status](https://ci.appveyor.com/api/projects/status/rgu44jqi1kt2jpw9?svg=true)](https://ci.appveyor.com/project/w23/opensource)
 
 OpenSource
 ==========