Add badge for AppVeyor

Author: thejohnfreeman

README.rst

@@ -8,11 +8,15 @@ Extensible methods for Python mimicking typeclasses in Haskell.
 
 .. image:: https://travis-ci.org/thejohnfreeman/python-typeclasses.svg?branch=master
    :target: https://travis-ci.org/thejohnfreeman/python-typeclasses
-   :alt: Build status
+   :alt: Build status: Linux and OSX
 
-.. image:: https://readthedocs.org/projects/python-typeclasses/badge/?version=latest
-   :target: https://python-typeclasses.readthedocs.io/
-   :alt: Documentation status
+.. image:: https://ci.appveyor.com/api/projects/status/github/thejohnfreeman/python-typeclasses?branch=master&svg=true
+   :target: https://ci.appveyor.com/project/thejohnfreeman/python-typeclasses
+   :alt: Build status: Windows
+
+.. .. image:: https://readthedocs.org/projects/python-typeclasses/badge/?version=latest
+..    :target: https://python-typeclasses.readthedocs.io/
+..    :alt: Documentation status
 
 .. image:: https://img.shields.io/pypi/v/typeclasses.svg
    :target: https://pypi.org/project/typeclasses/