Add badge for Appveyor tests.
.. image:: https://img.shields.io/travis/cherrypy/cherrypy/master.svg
:target: http://travis-ci.org/cherrypy/cherrypy
+.. image:: https://img.shields.io/appveyor/ci/gruntjs/grunt.svg
+ :target: https://ci.appveyor.com/project/jaraco/cherrypy
+
.. image:: https://img.shields.io/pypi/pyversions/cherrypy.svg
.. image:: https://img.shields.io/pypi/dm/cherrypy.svg
