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?

Start by reading the existing vim-codefmt documentation and the plugin's formatter integration points. Document how to write a lightweight plugin for a custom formatter, including the required interface and a working usage example. Done means a newcomer can follow the instructions to create and use such a plugin.

Written by the indexing model from the issue text.