Add example for writing an appservice

还没有人认领这个 Issue。

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](https://github.com/matrix-org/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).

审查 repository 中现有的 SDK 使用文档以及链接的 matrix-doc/issues/1144 上下文。确定 appservice 示例应放置的位置，并记录如何在不依赖 asyncio 的情况下为 appservice 使用 Python SDK。完成标准是：通过一个可用的示例说明传统使用场景及其变通方案。

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