add badge for windows test

Author: wing328

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/swagger-api/swagger-codegen.svg)](https://travis-ci.org/swagger-api/swagger-codegen)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-codegen-project/badge.svg?style=plastic)](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-codegen-project)
 [![PR Stats](http://issuestats.com/github/swagger-api/swagger-codegen/badge/pr)](http://issuestats.com/github/swagger-api/swagger-codegen) [![Issue Stats](http://issuestats.com/github/swagger-api/swagger-codegen/badge/issue)](http://issuestats.com/github/swagger-api/swagger-codegen)
+[![Windows Test](https://ci.appveyor.com/api/projects/status/github/swagger-api/swagger-codegen?branch=master&svg=true&passingText=Windows%20Test%20-%20OK&failingText=Windows%20Test%20-%20Fails)](#)
 
 :star::star::star: If you would like to contribute, please refer to [guidelines](https://github.com/swagger-api/swagger-codegen/blob/master/CONTRIBUTING.md) and a list of [open tasks](https://github.com/swagger-api/swagger-codegen/issues?q=is%3Aopen+is%3Aissue+label%3A%22Need+community+contribution%22).:star::star::star: