Docs: add mixin example

example/src/mixin.js

@@ -0,0 +1,25 @@
+/**
+ * This provides methods used for event handling. It's not meant to
+ * be used directly.
+ * @mixin
+ * @memberOf Geometry
+ */
+var Eventful = {
+    /**
+     * Register a handler function to be called whenever this event is fired.
+     * @param {string} eventName - Name of the event.
+     * @param {function(Object)} handler - The handler to call.
+     */
+    on: function(eventName, handler) {
+        // code...
+    },
+
+    /**
+     * Fire an event, causing all handlers for that event name to run.
+     * @param {string} eventName - Name of the event.
+     * @param {Object} eventData - The data provided to each handler.
+     */
+    fire: function(eventName, eventData) {
+        // code...
+    }
+};