add appveyor badge

Author: wholmgren

README.md

@@ -2,6 +2,7 @@ pvlib-python
 ============
 
 [![TravisCI](https://travis-ci.org/pvlib/pvlib-python.svg?branch=master)](https://travis-ci.org/pvlib/pvlib-python)
+[![Build status](https://ci.appveyor.com/api/projects/status/gr2eyhc84tvtkopk?svg=true)](https://ci.appveyor.com/project/wholmgren/pvlib-python-fv2to)
 [![Coverage Status](https://img.shields.io/coveralls/pvlib/pvlib-python.svg)](https://coveralls.io/r/pvlib/pvlib-python)
 [![Documentation Status](https://readthedocs.org/projects/pvlib-python/badge/?version=latest)](http://pvlib-python.readthedocs.org/en/latest/)
 [![DOI](https://zenodo.org/badge/doi/10.5281/zenodo.20562.svg)](http://dx.doi.org/10.5281/zenodo.20562)