Add AppVeyor badge to README (#328)

Fixes #325

Author: lurch

README.md

@@ -5,6 +5,7 @@ Python's Filesystem abstraction layer.
 [![PyPI version](https://badge.fury.io/py/fs.svg)](https://badge.fury.io/py/fs)
 [![PyPI](https://img.shields.io/pypi/pyversions/fs.svg)](https://pypi.org/project/fs/)
 [![Build Status](https://travis-ci.org/PyFilesystem/pyfilesystem2.svg?branch=master)](https://travis-ci.org/PyFilesystem/pyfilesystem2)
+[![Windows Build Status](https://ci.appveyor.com/api/projects/status/github/pyfilesystem/pyfilesystem2?branch=master&svg=true)](https://ci.appveyor.com/project/willmcgugan/pyfilesystem2)
 [![Coverage Status](https://coveralls.io/repos/github/PyFilesystem/pyfilesystem2/badge.svg)](https://coveralls.io/github/PyFilesystem/pyfilesystem2)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/30ad6445427349218425d93886ade9ee)](https://www.codacy.com/app/will-mcgugan/pyfilesystem2?utm_source=github.com&utm_medium=referral&utm_content=PyFilesystem/pyfilesystem2&utm_campaign=Badge_Grade)
 [![Code Health](https://landscape.io/github/PyFilesystem/pyfilesystem2/master/landscape.svg?style=flat)](https://landscape.io/github/PyFilesystem/pyfilesystem2/master)