[Reference] Add missing option delivery_whitelist

Q | A
--------
Doc fix? |	yes
New docs? | no
Applies to 	| SwiftMailerBundle 2.1
Fixed tickets | no

Author: TerjeBr

reference/configuration/swiftmailer.rst

@@ -32,6 +32,7 @@ Configuration
     * `threshold`_
     * `sleep`_
 * `delivery_address`_
+* `delivery_whitelist`_
 * `disable_delivery`_
 * `logging`_
 
@@ -156,6 +157,17 @@ emails sent during development go to a single account.
 This uses ``Swift_Plugins_RedirectingPlugin``. Original recipients are available
 on the ``X-Swift-To``, ``X-Swift-Cc`` and ``X-Swift-Bcc`` headers.
 
+delivery_whitelist
++~~~~~~~~~~~~~~~~~~
++
++**type**: ``array``
++
++Used in combination with ``delivery_address``. If set, emails matching any of these
+patterns will be delivered like normal, instead of being sent to `delivery_address`.
+For details, see `the Cookbook entry.`_
+
+..  _`the Cookbook entry.`: http://symfony.com/doc/current/cookbook/email/dev_environment.html#sending-to-a-specified-address-but-with-exceptions
+
 disable_delivery
 ~~~~~~~~~~~~~~~~