Add example for writing an appservice

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).

Examinez la documentation existante du dépôt concernant l’utilisation du SDK ainsi que le contexte lié de matrix-doc/issues/1144. Identifiez où placer un exemple d’appservice et documentez comment utiliser le Python SDK pour un appservice sans dépendre d’asyncio. Le travail est terminé lorsque le scénario d’utilisation traditionnel et sa solution de contournement sont expliqués avec un exemple utilisable.