Clarified how to load translations from db

Added link the the translation loader tag documentation.

Author: Epskampie

book/translation.rst

@@ -332,7 +332,8 @@ taste.
 
     You can also store translations in a database, or any other storage by
     providing a custom class implementing the
-    :class:`Symfony\\Component\\Translation\\Loader\\LoaderInterface` interface.
+    :class:`Symfony\\Component\\Translation\\Loader\\LoaderInterface` interface. 
+    See the `translation loader tag`_ for more information.
 
 .. index::
    single: Translations; Creating translation resources
@@ -1013,3 +1014,4 @@ steps:
 .. _`Translatable Extension`: https://github.com/l3pp4rd/DoctrineExtensions
 .. _`ISO3166 Alpha-2`: http://en.wikipedia.org/wiki/ISO_3166-1#Current_codes
 .. _`ISO639-1`: http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
+.. _`translation loader tag`: http://symfony.com/doc/current/reference/dic_tags.html#translation-loader