add validation config reference section

Author: xabbuh

reference/configuration/framework.rst

@@ -48,6 +48,11 @@ Configuration
 * `translator`_
     * :ref:`enabled <translator.enabled>`
     * `fallback`_
+* `validation`_
+    * `cache`_
+    * `enable_annotations`_
+    * `translation_domain`_
+    * `strict_email`_
 
 secret
 ~~~~~~
@@ -517,6 +522,40 @@ This option is used when the translation key for the current locale wasn't found
 
 For more details, see :doc:`/book/translation`.
 
+validation
+~~~~~~~~~~
+
+cache
+.....
+
+**type**: ``string``
+
+The implementation to be used to cache validation metadata.
+
+enable_annotations
+..................
+
+**type**: ``Boolean`` **default**: ``false``
+
+If this option is enabled, validation constraints can be defined using annotations.
+
+translation_domain
+..................
+
+**type**: ``string`` **default**: ``validators``
+
+The translation domain that is used when translating validation constraint
+error messages.
+
+strict_email
+............
+
+**type**: ``Boolean`` **default**: ``false``
+
+If this option is enabled, the `egulias/email-validator`_ library will be
+used by the :doc:`/reference/constraints/Email` constraint validator. Otherwise,
+the validator uses a simple regular expression to validate email addresses.
+
 Full default Configuration
 --------------------------
 
@@ -646,3 +685,4 @@ Full default Configuration
 
 .. _`protocol-relative`: http://tools.ietf.org/html/rfc3986#section-4.2
 .. _`PhpStormOpener`: https://github.com/pinepain/PhpStormOpener
+.. _`egulias/email-validator`: https://github.com/egulias/EmailValidator