Improve Symfony bundle documentation (#23)

docs/symfony.md

@@ -35,10 +35,42 @@ Possible properties:
 - `cache_dir` (default: `%kernel.cache_dir%/automapper`): This setting allows you to customize the output directory 
 for generated mappers;
 - `mappings`: This option allows you to customize Mapper metadata, you have to specify `source` & `target` data types 
-and related configuration using `pass` field. This configuration should implements `AutoMapper\Bundle\Configuration\ConfigurationPassInterface`.
+and related configuration using `pass` field. This configuration should implements `AutoMapper\Bundle\Configuration\MapperConfigurationInterface`.
 - `allow_readonly_target_to_populate` (default: `false`): Will throw an exception if you use a readonly class as target 
 to populate if set to `false`.
 
+### MapperConfigurationInterface
+
+You can add some metadata to customize a transformation. Here is an example:
+
+```php
+class UserMapperConfiguration implements MapperConfigurationInterface
+{
+    public function getSource(): string
+    {
+        return User::class;
+    }
+
+    public function getTarget(): string
+    {
+        return UserDTO::class;
+    }
+
+    public function process(MapperGeneratorMetadataInterface $metadata): void
+    {
+        $metadata->forMember('yearOfBirth', function (User $user) {
+            return ((int) date('Y')) - ((int) $user->age);
+            return ((int) date('Y')) - ((int) $user->age);
+        });
+    }
+}
+```
+
+Here you have to inherit the `MapperConfigurationInterface` interface, that ways it will autoconfigure your 
+transformation for a given source & target. And in the `process` method you will add or overwrite fields to map.
+The fields you give onto the `forMember` method are target fields and the callback will always give you the source 
+object.
+
 ### Normalizer Bridge 🌁
 A Normalizer Bridge is available, aiming to be 100% feature compatible with the ObjectNormalizer of the 
 ``symfony/serializer`` component. The goal of this bridge **is not to replace the ObjectNormalizer** but rather