add versionadded directive for multiple option

Author: xabbuh

reference/forms/types/file.rst

@@ -86,6 +86,9 @@ Field Options
 multiple
 ~~~~~~~~
 
+.. versionadded:: 2.5
+    The ``multiple`` option was introduced in Symfony 2.5.
+
 **type**: ``Boolean`` **default**: ``false``
 
 When set to true, the user will be able to upload multiple files at the same time.