✨ Add docs/tests extending OpenAPI (#126)

docs/tutorial/extending-openapi.md

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+!!! warning
+    This is a rather advanced feature. You probably can skip it.
+
+    If you are just following the tutorial - user guide, you can probably skip this section.
+
+    If you already know that you need to modify the generated OpenAPI schema, continue reading.
+
+
+There are some cases where you might need to modify the generated OpenAPI schema.
+
+In this section you will see how.
+
+## The normal process
+
+The normal (default) process, is as follows.
+
+A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.
+
+As part of the application object creation, a *path operation* for `/openapi.json` (or for whatever you set your `openapi_url`) is registered.
+
+It just returns a JSON response with the result of the application's `.openapi()` method.
+
+By default, what the method `.openapi()` does is check the property `.openapi_schema` to see if it has contents and return them.
+
+If it doesn't, it generates them using the utility function at `fastapi.openapi.utils.get_openapi`.
+
+And that function `get_openapi()` receives as parameters:
+
+* `title`: The OpenAPI title, shown in the docs.
+* `version`: The version of your API, e.g. `2.5.0`.
+* `openapi_version`: The version of the OpenAPI specification used. By default, the latest: `3.0.2`.
+* `description`: The description of your API.
+* `routes`: A list of routes, these are each of the registered *path operations*. They are taken from `app.routes`.
+* `openapi_prefix`: The URL prefix to be used in your OpenAPI.
+
+## Overriding the defaults
+
+Using the information above, you can use the same utility function to generate the OpenAPI schema and override each part that you need.
+
+For example, let's add <a href="https://github.com/Rebilly/ReDoc/blob/master/docs/redoc-vendor-extensions.md#x-logo" target="_blank">ReDoc's OpenAPI extension to include a custom logo</a>.
+
+### Normal **FastAPI**
+
+First, write all your **FastAPI** application as normally:
+
+```Python hl_lines="1 4 7 8 9"
+{!./src/extending_openapi/tutorial001.py!}
+```
+
+### Generate the OpenAPI schema
+
+Then, use the same utility function to generate the OpenAPI schema, inside a `custom_openapi()` function:
+
+```Python hl_lines="2 15 16 17 18 19 20"
+{!./src/extending_openapi/tutorial001.py!}
+```
+
+### Modify the OpenAPI schema
+
+Now you can add the ReDoc extension, adding a custom `x-logo` to the `info` "object" in the OpenAPI schema:
+
+```Python hl_lines="21 22 23"
+{!./src/extending_openapi/tutorial001.py!}
+```
+
+### Cache the OpenAPI schema
+
+You can use the property `.openapi_schema` as a "cache", to store your generated schema.
+
+That way, your application won't have to generate the schema every time a user opens your API docs.
+
+It will be generated only once, and then the same cached schema will be used for the next requests.
+
+```Python hl_lines="13 14 24 25"
+{!./src/extending_openapi/tutorial001.py!}
+```
+
+### Override the method
+
+Now you can replace the `.openapi()` method with your new function.
+
+```Python hl_lines="28"
+{!./src/extending_openapi/tutorial001.py!}
+```
+
+### Check it
+
+Once you go to <a href="http://127.0.0.1:8000/redoc" target="_blank">http://127.0.0.1:8000/redoc</a> you will see that you are using your custom logo (in this example, **FastAPI**'s logo):
+
+<img src="/img/tutorial/extending-openapi/image01.png">