docs(README): add an appveyor badge

Author: pine

README.md

@@ -3,6 +3,7 @@ ArrayBuffer loader for webpack
 
 [![npm](https://img.shields.io/npm/v/arraybuffer-loader.svg?maxAge=2592000&style=flat-square)](https://www.npmjs.org/package/arraybuffer-loader)
 [![Travis](https://img.shields.io/travis/pine/arraybuffer-loader/master.svg?maxAge=2592000&style=flat-square)](https://travis-ci.org/pine/arraybuffer-loader)
+[![Build Status](https://img.shields.io/appveyor/ci/pine/arraybuffer-loader/master.svg?style=flat-square&maxAge=2592000)](https://ci.appveyor.com/project/pine/arraybuffer-loader/branch/master)
 [![Dependency Status](https://img.shields.io/david/pine/arraybuffer-loader.svg?maxAge=2592000&style=flat-square)](https://david-dm.org/pine/arraybuffer-loader)
 [![devDependency Status](https://img.shields.io/david/dev/pine/arraybuffer-loader.svg?maxAge=2592000&style=flat-square)](https://david-dm.org/pine/arraybuffer-loader?type=dev)