Update readme: How to register type if I use Doctrine directly

[dimabory]

Adopting this code:

require_once __DIR__.'/../vendor/autoload.php';

use Goodwix\DoctrineJsonOdm\Type\ODMType;
use Symfony\Component\Serializer\SerializerInterface;

class Document { }

ODMType::registerODMType(
    Document::class,
    new class implements SerializerInterface
    {
        public function serialize($data, $format, array $context = [])  { /* Implement serialize() method. */ }
        public function deserialize($data, $type, $format, array $context = [])  { /* Implement deserialize() method. */ }
    }
);

[strider2038]

@dimabory thanks, we added a description to readme https://github.com/goodwix/doctrine-json-odm#manual-usage-with-doctrine-and-symfony-serializer-component