Add black box JS extension docstring

Author: vidartf

js/src/objects/Blackbox.js

@@ -1,6 +1,26 @@
 var _ = require('underscore');
 var BlackboxAutogen = require('./Blackbox.autogen').BlackboxModel;
 
+/**
+ * This object is intended as an extension point. It differs from
+ * a normal Object3D in that it does not sync back child objects to
+ * the models. This allows extensions to build sub-scene graphs that
+ * might not be possible/feasible/desirable to sync.
+ *
+ * To implement an extended object based on this:
+ *  - Extend this model
+ *  - Override defaults/serializers as normal for a widget. Remember
+ *    to call the super methods!
+ *  - Override initialize method. Call super, and then put any
+ *    extension code that relies on the THREE object inside
+ *    a "then" to `initPromise:
+ *
+ *      this.initPromise.then(() => {
+ *        // Do your stuff here
+ *      });
+ *  - If you do not want a basic Object3D as the "root" of your black
+ *    box, you need to override constructThreeObject[Async].
+ */
 var BlackboxModel = BlackboxAutogen.extend({
 
     defaults: function() {