add informations how to create a custom doctrine mapping

i tryed to explain a bit how doctrine tries to find the correct mapping, the driver and how you can configure your own mapping.
missing XML:
````xml
<?xml version="1.0" charset="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:doctrine="http://symfony.com/schema/dic/doctrine">

    <doctrine:config>
        <doctrine:orm auto-mapping="true">
            <mapping name="AppBundle" dir="Resources/config/doctrine" />
        </doctrine:orm>
    </doctrine:config>
</container>
````

and PHP:
````php
$container->loadFromExtension('doctrine', array(
    'orm' => array(
        'auto_mapping' => true,
        'mappings' => array(
            'AppBundle' => array('dir' => 'Resources/config/doctrine'),
        ),
    ),
));
````

I would also like an explanation of the path, is this relative to the bundle or to the app kernel dir, etc.