add AppVeyor badge

Author: dlfivefifty

README.md

@@ -4,6 +4,7 @@ Core functionality of ApproxFun
 [![](https://img.shields.io/badge/docs-stable-blue.svg)](https://JuliaApproximation.github.io/ApproxFun.jl/stable)
 [![](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaApproximation.github.io/ApproxFun.jl/latest)
 [![Build Status](https://travis-ci.org/JuliaApproximation/ApproxFun.jl.svg?branch=master)](https://travis-ci.org/JuliaApproximation/ApproxFunBase.jl) 
+[![Build status](https://ci.appveyor.com/api/projects/status/q3e3ihfbxnrjn7ji?svg=true)](https://ci.appveyor.com/project/dlfivefifty/approxfunbase-jl)
 [![Coverage Status](https://img.shields.io/coveralls/JuliaApproximation/ApproxFunBase.jl.svg)](https://coveralls.io/r/JuliaApproximation/ApproxFunBase.jl?branch=master) 
 [![Join the chat at https://gitter.im/JuliaApproximation/ApproxFun.jl](https://badges.gitter.im/JuliaApproximation/ApproxFun.jl.svg)](https://gitter.im/JuliaApproximation/ApproxFun.jl?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)