Add "versionadded" directive for 2.5

Author: bicpi

book/validation.rst

@@ -586,6 +586,9 @@ allows you to add a constraint to any public method whose name starts with
 "get", "is" or "has". In this guide, these types of methods are referred to
 as "getters".
 
+.. versionadded:: 2.5
+    Support for methods starting with ``has`` is new in Symfony 2.5.
+
 The benefit of this technique is that it allows you to validate your object
 dynamically. For example, suppose you want to make sure that a password field
 doesn't match the first name of the user (for security reasons). You can