Added Translating Validation Messages to cookbook.

doc/cookbook/translating_validation_messages.rst

+Translating Validation Messages
+===============================
+
+When working with Symfony2 forms, a common task would be to show localized validation messages.
+
+In order to do that, you will need to register translator and point to translated resources:
+
+::
+
+    $app->register(new Silex\Provider\TranslationServiceProvider(), array(
+        'locale' => 'sr_Latn',
+        'translation.class_path' => __DIR__ . '/vendor/symfony/src',
+        'translator.messages' => array()
+    ));
+    $app['translator']->addLoader('xlf', new Symfony\Component\Translation\Loader\XliffFileLoader());
+    $app['translator']->addResource('xlf', __DIR__ . '/vendor/symfony/src/Symfony/Bundle/FrameworkBundle/Resources/translations/validators.sr_Latn.xlf', 'sr_Latn', 'validators');
+
+And that's all you need to load translations from Symfony2 ``xlf`` files.
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