[Sortable] Move an item at the end of the list

See also: http://stackoverflow.com/questions/14688469/ordering-entities-with-sortable-behavior-in-doctrine-2-symfony-2/20092238#20092238

I've added to the Sortable documentation how to move an item to the end of the list.

However I have not tested this behavior, can you confirm that this is correct?
Merge pull request #907 from mnapoli/patch-1

[Sortable] Move an item at the end of the list