docs(readme): document how to make undocumented requests (#407)

README.md

@@ -448,6 +448,41 @@ with client.messages.with_streaming_response.create(
 
 The context manager is required so that the response will reliably be closed.
 
+### Making custom/undocumented requests
+
+This library is typed for convenient access the documented API.
+
+If you need to access undocumented endpoints, params, or response properties, the library can still be used.
+
+#### Undocumented endpoints
+
+To make requests to undocumented endpoints, you can make requests using `client.get`, `client.post`, and other
+http verbs. Options on the client will be respected (such as retries) will be respected when making this
+request.
+
+```py
+import httpx
+
+response = client.post(
+    "/foo",
+    cast_to=httpx.Response,
+    body={"my_param": True},
+)
+
+print(response.headers.get("x-foo"))
+```
+
+#### Undocumented params
+
+If you want to explicitly send an extra param, you can do so with the `extra_query`, `extra_body`, and `extra_headers` request
+options.
+
+#### Undocumented properties
+
+To access undocumented response properties, you can access the extra fields like `response.unknown_prop`. You
+can also get all the extra fields on the Pydantic model as a dict with
+[`response.model_extra`](https://docs.pydantic.dev/latest/api/base_model/#pydantic.BaseModel.model_extra).
+
 ### Configuring the HTTP client
 
 You can directly override the [httpx client](https://www.python-httpx.org/api/#client) to customize it for your use case, including: