Add AppVeyor build status

Author: chrisdone

README.md

@@ -1,4 +1,4 @@
-# jl [![Build Status](https://travis-ci.org/chrisdone/jl.png)](https://travis-ci.org/chrisdone/jl)
+# jl [![Build Status](https://travis-ci.org/chrisdone/jl.png)](https://travis-ci.org/chrisdone/jl) [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/chrisdone/path?svg=true)](https://ci.appveyor.com/project/chrisdone/jl)
 
 jl ("JSON lambda") is a tiny functional language for querying and
 manipulating JSON.
@@ -14,6 +14,9 @@ $ jl 'map $ \o -> { sha: o.sha, ps: map _.sha o.parents }' x.json
 
 Binary releases for Linux and OS X are available [here](https://github.com/chrisdone/jl/releases).
 
+Builds on Windows (see AppVeyor status), haven't added Windows
+binaries to the releases yet.
+
 Installing from source:
 
 1. Get [stack](https://haskell-lang.org/get-started)