docs(parser): add JSON string field extension example

docs/utilities/parser.md

@@ -171,7 +171,7 @@ Parser comes with the following built-in models:
 | **KafkaSelfManagedEventModel**  | Lambda Event Source payload for self managed Kafka payload         |
 | **KafkaMskEventModel**          | Lambda Event Source payload for AWS MSK payload                    |
 
-### extending built-in models
+### Extending built-in models
 
 You can extend them to include your own models, and yet have all other known fields parsed along the way.
 
@@ -236,6 +236,19 @@ for order_item in ret.detail.items:
 3. Defined how part of our EventBridge event should look like by overriding `detail` key within our `OrderEventModel`
 4. Parser parsed the original event against `OrderEventModel`
 
+???+ tip
+    When extending a `string` field containing JSON, you need to wrap the field
+    with [Pydantic's Json Type](https://pydantic-docs.helpmanual.io/usage/types/#json-type):
+
+    ```python
+    from pydantic import Json
+
+    ...
+
+    class OrderEventModel(APIGatewayProxyEventV2Model):
+        body: Json[Order]
+    ```
+
 ## Envelopes
 
 When trying to parse your payloads wrapped in a known structure, you might encounter the following situations: