README: add AppVeyor badge

Author: Jorge Aparicio

README.md

@@ -1,6 +1,8 @@
-[![Build Status][status]](https://travis-ci.org/japaric/rustc-builtins)
+[![Travis][travis]](https://travis-ci.org/japaric/rustc-builtins)
+[![AppVeyor][appveyor]](https://ci.appveyor.com/project/japaric/rustc-builtins)
 
-[status]: https://travis-ci.org/japaric/rustc-builtins.svg?branch=master
+[travis]: https://travis-ci.org/japaric/rustc-builtins.svg?branch=master
+[appveyor]: https://ci.appveyor.com/api/projects/status/mofi55r3px9pv3o7?svg=true
 
 # `rustc-builtins`