add appveyor badge to the readme

Author: tkelman

README.md

@@ -1,6 +1,7 @@
 ## LightXML.jl
 
 [![Build Status](https://travis-ci.org/JuliaLang/LightXML.jl.svg?branch=master)](https://travis-ci.org/JuliaLang/LightXML.jl)
+[![Build status](https://ci.appveyor.com/api/projects/status/si07guw1bbn4srtw/branch/master?svg=true)](https://ci.appveyor.com/project/tkelman/lightxml-jl/branch/master)
 
 This package is a light-weight Julia wrapper of [libxml2](http://www.xmlsoft.org), which provides a minimal interface that covers functionalities that are commonly needed: