Added docs about the format option

Author: dosten

reference/constraints/DateTime.rst

@@ -3,12 +3,13 @@ DateTime
 
 Validates that a value is a valid "datetime", meaning either a ``DateTime``
 object or a string (or an object that can be cast into a string) that follows
-a valid YYYY-MM-DD HH:MM:SS format.
+a specific format.
 
 +----------------+------------------------------------------------------------------------+
 | Applies to     | :ref:`property or method <validation-property-target>`                 |
 +----------------+------------------------------------------------------------------------+
-| Options        | - `message`_                                                           |
+| Options        | - `format`_                                                            |
+|                | - `message`_                                                           |
 |                | - `payload`_                                                           |
 +----------------+------------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\DateTime`          |
@@ -78,6 +79,17 @@ Basic Usage
 Options
 -------
 
+format
+~~~~~~
+
+**type**: ``string`` **default**: ``Y-m-d H:i:s``
+
+.. versionadded:: 3.1
+    The ``format`` option was introduced in Symfony 3.1.
+
+This option allows to validate a custom date format.
+See `DateTime::createFromFormat <date_create_from_format>`_ for formatting options.
+
 message
 ~~~~~~~
 
@@ -86,3 +98,5 @@ message
 This message is shown if the underlying data is not a valid datetime.
 
 .. include:: /reference/constraints/_payload-option.rst.inc
+
+.. _`date_create_from_format`: http://php.net/manual/en/datetime.createfromformat.php