✨ Add docs about responses with additional status codes (#156)

* ✨ Add docs about responses with additional status codes

* 📝 Update docs, link to documenting additional responses

docs/tutorial/additional-status-codes.md

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+By default, **FastAPI** will return the responses using Starlette's `JSONResponse`, putting the content you return from your *path operation* inside of that `JSONResponse`.
+
+It will use the default status code or the one you set in your *path operation*.
+
+## Additional status codes
+
+If you want to return additional status codes apart from the main one, you can do that by returning a `Response` directly, like a `JSONResponse`, and set the additional status code directly.
+
+For example, let's say that you want to have a *path operation* that allows to update items, and returns HTTP status codes of 200 "OK" when successful.
+
+But you also want it to accept new items. And when the items didn't exist before, it creates them, and returns an HTTP status code of 201 "Created".
+
+To achieve that, import `JSONResponse`, and return your content there directly, setting the `status_code` that you want:
+
+```Python hl_lines="2 20"
+{!./src/additional_status_codes/tutorial001.py!}
+```
+
+!!! warning
+    When you return a `Response` directly, like in the example above, it will be returned directly.
+
+    It won't be serialized with a model, etc.
+    
+    Make sure it has the data you want it to have, and that the values are valid JSON (if you are using `JSONResponse`).
+
+## OpenAPI and API docs
+
+If you return additional status codes and responses directly, they won't be included in the OpenAPI schema (the API docs), because FastAPI doesn't have a way to know before hand what you are going to return.
+
+But you can document that in your code, using: <a href="https://fastapi.tiangolo.com/tutorial/additional-responses/" target="_blank">Additional Responses</a>.