Add AppVeyor badge to README

Author: Mikolaj

README.md

@@ -2,3 +2,4 @@
 [![Hackage](https://img.shields.io/hackage/v/threadscope.svg)](https://hackage.haskell.org/package/threadscope)
 [![Hackage-Deps](https://img.shields.io/hackage-deps/v/threadscope.svg)](http://packdeps.haskellers.com/feed?needle=threadscope)
 [![Build Status](https://travis-ci.org/haskell/ThreadScope.svg?branch=master)](https://travis-ci.org/haskell/ThreadScope)
+[![Build status](https://ci.appveyor.com/api/projects/status/y6kd8pyh2f3qok4f?svg=true)](https://ci.appveyor.com/project/Mikolaj/threadscope)