add appveyor badge

Author: alimanfoo

README.rst

@@ -8,3 +8,6 @@ more information.
 
 .. image:: https://travis-ci.org/alimanfoo/zarr.svg?branch=master
     :target: https://travis-ci.org/alimanfoo/zarr
+
+.. image:: https://ci.appveyor.com/api/projects/status/7d4iko59k8hpbbik?svg=true
+    :target: https://ci.appveyor.com/project/alimanfoo/zarr