[README] Add appveyor badge

Author: mpsonntag

README.rst

@@ -1,10 +1,13 @@
-odML libraries and editor
-=========================
 .. image:: https://travis-ci.org/G-Node/python-odml.svg?branch=master
     :target: https://travis-ci.org/G-Node/python-odml
+.. image:: https://ci.appveyor.com/api/projects/status/2wfvsu7boe18kwjy?svg=true
+    :target: https://ci.appveyor.com/project/mpsonntag/python-odml
 .. image:: https://coveralls.io/repos/github/G-Node/python-odml/badge.svg?branch=master
     :target: https://coveralls.io/github/G-Node/python-odml?branch=master
 
+odML libraries and editor
+=========================
+
 Dependencies
 ------------