docs : add note about argument within addItems

_posts/docs/2010-12-04-methods.mdown

@@ -32,7 +32,7 @@ $('#container').isotope( 'methodName', [optionalParameters] )
 
 {% endhighlight %}
 
-Adds elements to the pool of item elements of the container, but does sort, filter or layout. See [Adding items](adding-items.html) for more details.
+Adds elements to the pool of item elements of the container, but does sort, filter or layout. See [Adding items](adding-items.html) for more details. The argument within the callback is the group of elements that were added. 
 
 [**See Demo: Adding items**](../demos/adding-items.html)