Adding appveyor badge

README.md

@@ -1,3 +1,4 @@
+[![Build status](https://ci.appveyor.com/api/projects/status/xh2lsliy6usk60o5?svg=true)](https://ci.appveyor.com/project/SebastienRos/jint)
 [![NuGet](https://img.shields.io/nuget/v/Jint.svg)](https://www.nuget.org/packages/Jint)
 [![Join the chat at https://gitter.im/sebastienros/jint](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/sebastienros/jint)