Document opts.encodings usage

Hi, I would like to provide my own encoder & decoder for https://github.com/mapbox/vector-tile-spec/blob/master/2.1/vector_tile.proto#L17 (Value message) to improve efficiency for my usecase, but got a bit confusing in all the codegen logic. Please provide a sample of how to use `opts.encodings` to override a single message parser/generator. Thanks!

Start by locating the opts.encodings option and following the code-generation path for message parsers and generators. Document a sample that overrides the parser and generator for one message, using the Value message as the example; done means a newcomer can understand and adapt the example.

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