Add example for writing an appservice

[Torxed]

Since this repo is officially still open under the matrix-org account, I would suggest to update any documentation regarding using the SDK libraries for appservice purposes. Since matrix-doc/issues/1144 broke the traditional MatrixClient(url, token, user_id) usage scenario and there's very little information on how to apply this SDK to work around it, it would be nice with some updated documentation.

(And I know about the nio recommendation, but for people that don't want/need/can use the asyncio library, this would be a good addition).