Add Appveyor build badge

Author: smashwilson

README.md

@@ -1,6 +1,7 @@
 # Atom GitHub Package
 
 [![Build Status](https://travis-ci.com/atom/github.svg?token=RwrCnzpsZN5oEq5S5p7V&branch=master)](https://travis-ci.com/atom/github)
+[![Build status](https://ci.appveyor.com/api/projects/status/psctk8vrva49dseb/branch/master?svg=true)](https://ci.appveyor.com/project/Atom/github/branch/master)
 
 The Atom GitHub package provides Git and GitHub integration for Atom.