add link to project wiki in README.md (#9987)

README.md

@@ -5,8 +5,11 @@ three.js
 
 The aim of the project is to create an easy to use, lightweight, 3D library. The library provides <canvas>, <svg>, CSS3D and WebGL renderers.
 
-[Examples](http://threejs.org/examples/) — [Documentation](http://threejs.org/docs/) — [Migrating](https://github.com/mrdoob/three.js/wiki/Migration) — [Help](http://stackoverflow.com/questions/tagged/three.js)
-
+[Examples](http://threejs.org/examples/) —
+[Documentation](http://threejs.org/docs/) —
+[Wiki](https://github.com/mrdoob/three.js/wiki) —
+[Migrating](https://github.com/mrdoob/three.js/wiki/Migration) —
+[Help](http://stackoverflow.com/questions/tagged/three.js)
 
 ### Usage ###