Add Appveyor badge for Windows CI.

README.rst

@@ -716,6 +716,9 @@ User support  https://stackoverflow.com/search?q=cfgrib
 Code quality  .. image:: https://api.travis-ci.org/ecmwf/cfgrib.svg?branch=master
                 :target: https://travis-ci.org/ecmwf/cfgrib/branches
                 :alt: Build Status on Travis CI
+              .. image:: https://ci.appveyor.com/api/projects/status/github/ecmwf/cfgrib?svg=true&passingText=passing&failingText=failing&pendingText=pending
+                :target: https://ci.appveyor.com/project/StephanSiemen/cfgrib
+                :alt: Build Status on Appveyor
               .. image:: https://coveralls.io/repos/ecmwf/cfgrib/badge.svg?branch=master&service=github
                 :target: https://coveralls.io/github/ecmwf/cfgrib
                 :alt: Coverage Status on Coveralls