Add appveyor badge

Author: acoburn

README.md

@@ -4,6 +4,7 @@ A simple way to expose static assets as a read-only <a href="https://www.w3.org/
 
 [![Minimum PHP Version](https://img.shields.io/badge/php-%3E%3D%205.6-8892BF.svg?style=flat-square)](https://php.net/)
 [![Build Status](https://travis-ci.org/trellis-ldp/static-ldp.svg?branch=master)](https://travis-ci.org/trellis-ldp/static-ldp)
+[![Build status](https://ci.appveyor.com/api/projects/status/pad3wdgd43lg1nou?svg=true)](https://ci.appveyor.com/project/acoburn/static-ldp)
 
 ## Requirements