1.12 Upgrade Guide: Document widget mixins

Author: scottgonzalez

page/upgrade-guide/1.12.md

@@ -473,6 +473,27 @@ $( "#dialog" ).dialog( {
 
 This will add the `ui-corner-top` and `awesome-fade-class` classes to the element which has the `ui-dialog` class, while also removing the default `ui-corner-all` class from the element which has the `ui-dialog-titlebar` class.
 
+### Added support for widget mixins
+
+[(#12601)](http://bugs.jqueryui.com/ticket/12601) The widget factory now supports mixins so that you can have common behaviors shared across many widgets in scenarios where inheritance doesn't make sense. Until now, this required manually extending the widget prototype with the mixins. Under the hood, this is all that's happening, but there's now dedicated syntax to make calls to the widget factory a little cleaner.
+
+Old:
+```js
+$.widget( "demo.widget", $.extend( {
+
+	// Widget prototype
+}, someMixin, someOtherMixin ) );
+```
+
+New:
+$.widget( "demo.widget", [ someMixin, someOtherMixin ], {
+
+	// Widget prototype
+} );
+```
+
+As you can see, the new syntax makes it more obvious that mixins are being used because they're at the top of the call, rather than at the bottom.
+
 ## Effects
 
 ### Converted transfer effect to a jQuery plugin method