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.
````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>
            <mapping name="Post" 
                type="annotation"
                dir="%kernel.root_dir%/../src/Entity"
                is-bundle="false"
                prefix="App\Entity"
                alias="App"
            />
        </doctrine:orm>
    </doctrine:config>
</container>
````

````php
$container->loadFromExtension('doctrine', array(
    'orm' => array(
        'auto_mapping' => true,
        'mappings' => array(
            'AppBundle' => array(
                'type'      => 'annotation',
                'dir'       => '%kernel.root_dir%/../src/Entity',
                'is_bundle' => false,
                'prefix'    => 'App\Entity',
                'alias'     => 'App',
            ),
        ),
    ),
));
````