added doc for collections js events.

Resources/doc/reference/form_types.rst

@@ -20,6 +20,7 @@ sonata_type_collection
 ^^^^^^^^^^^^^^^^^^^^^^
 
 The ``Collection Type`` is meant to handle creation and edition of model collections. Rows can be added and deleted, and your model abstraction layer may allow you to edit fields inline.
+**TIP**: A jQuery event is fired after a row has been added(*sonata-collection-item-added*) or deleted(*sonata-collection-item-deleted*). You can bind to them to trigger some custom javascript imported into your templates(eg: add a calendar widget to a just added date field)
 
 Type configuration
 ^^^^^^^^^^^^^^^^^^