Add appveyor badge to README

README.md

@@ -1,5 +1,6 @@
 [![Version](http://img.shields.io/packagist/v/tenside/core.svg?style=flat-square)](https://packagist.org/packages/tenside/core)
-[![Stable Build Status](http://img.shields.io/travis/tenside/core/master.svg?style=flat-square&label=stable build)](https://travis-ci.org/tenside/core)
+[![Stable Build Status Linux](http://img.shields.io/travis/tenside/core/master.svg?style=flat-square&label=stable%20build%20%28Linux%29)](https://travis-ci.org/tenside/core)
+[![Stable Build Status Windows](https://img.shields.io/appveyor/ci/tenside/core/master.svg?style=flat-square&label=stable%20build%20%28Windows%29)](https://ci.appveyor.com/project/tenside/core)
 [![Scrutinizer](https://img.shields.io/scrutinizer/g/tenside/core.svg?style=flat-square)](https://scrutinizer-ci.com/g/tenside/core/)
 [![License](http://img.shields.io/packagist/l/tenside/core.svg?style=flat-square)](http://spdx.org/licenses/MIT)
 [![Downloads](http://img.shields.io/packagist/dt/tenside/core.svg?style=flat-square)](https://packagist.org/packages/tenside/core)