Add Windows (appveyor) badge

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/urfave/cli.svg?branch=master)](https://travis-ci.org/urfave/cli)
+[![Windows Build Status](https://ci.appveyor.com/api/projects/status/rtgk5xufi932pb2v?svg=true)](https://ci.appveyor.com/project/meatballhat/cli)
 [![GoDoc](https://godoc.org/github.com/urfave/cli?status.svg)](https://godoc.org/github.com/urfave/cli)
 [![codebeat](https://codebeat.co/badges/0a8f30aa-f975-404b-b878-5fab3ae1cc5f)](https://codebeat.co/projects/github-com-urfave-cli)
 [![Go Report Card](https://goreportcard.com/badge/urfave/cli)](https://goreportcard.com/report/urfave/cli)