Better instructions for writing a plugin with a custom formatter

Maybe I've missed it, but it's not obvious to me how to write a plugin that plays nice with `vim-codefmt`. We have a custom code formatter at work, and I'd really like to write a lightweight internal plugin that uses codefmt to do the heavy interface lifting.

Any tips?

首先阅读现有的 vim-codefmt 文档以及插件的 formatter 集成点。记录如何为自定义 formatter 编写轻量级插件，包括所需的接口和一个可运行的使用示例。当新贡献者能够按照说明创建并使用此类插件时，即视为完成。

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