adding logo for app.json for heroku button

Author: craigkerstiens

app.json

@@ -3,6 +3,7 @@
   "description": "Code from the React tutorial",
   "keywords": [ "react", "reactjs", "tutorial" ],
   "repository": "https://github.com/reactjs/react-tutorial",
+  "logo": "https://facebook.github.io/react/img/logo.svg",
   "website": "http://facebook.github.io/react/docs/tutorial.html",
   "success_url": "/",
   "env" : {