Add Legacy Note (#71)

Author: darthtrevino

README.md

@@ -4,7 +4,9 @@
 [![devDependencies Status](https://david-dm.org/react-dnd/react-dnd-html5-backend/dev-status.svg)](https://david-dm.org/react-dnd/react-dnd-html5-backend?type=dev)
 [![peerDependencies Status](https://david-dm.org/react-dnd/react-dnd-html5-backend/peer-status.svg)](https://david-dm.org/react-dnd/react-dnd-html5-backend?type=peer)
 
-# React DnD HTML5 Backend
+# React DnD HTML5 Backend (LEGACY REPOSITORY)
+
+** NOTE: React-Dnd has been restructured as a monorepo. This code has been merged into the the [react-dnd](http://github.com/react-dnd/react-dnd) main repository.
 
 The officially supported HTML5 backend for [React DnD](http://react-dnd.github.io/react-dnd/).
 See [the docs](http://react-dnd.github.io/react-dnd/docs-html5-backend.html) for usage information.