Add AppVeyor badge.

Author: octalmage

README.md

@@ -2,6 +2,7 @@
 ========
 
 [![Build Status](https://api.travis-ci.org/octalmage/robotjs.svg?branch=master)](https://travis-ci.org/octalmage/robotjs) [![Join the chat at https://gitter.im/octalmage/robotjs](https://img.shields.io/badge/gitter-join%20chat-brightgreen.svg)](https://gitter.im/octalmage/robotjs?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Build Status](https://api.travis-ci.org/octalmage/robotjs.svg?branch=master)](https://travis-ci.org/octalmage/robotjs) [![Build status](https://ci.appveyor.com/api/projects/status/qh2eqb37j7ap6x36?svg=true)](https://ci.appveyor.com/project/octalmage/robotjs) 
 [![Wat: Documented](https://img.shields.io/badge/wat-documented-blue.svg)](https://github.com/dthree/wat)
 [![npm version](https://img.shields.io/npm/v/robotjs.svg)](https://www.npmjs.com/package/robotjs) 
 [![Ready](https://badge.waffle.io/octalmage/robotjs.svg?label=ready&title=Ready)](http://waffle.io/octalmage/robotjs)