docs: add protobuf

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[threexc]

I wonder if it's worth adding a comment: in the docs about this part of the workflow:

    # Unlike upstream (which builds a large cross-compiled matrix via Bazel), we
		
    # build the riscv64 bdist from the sdist with cibuildwheel. protobuf's
		
    # setup.py compiles the bundled upb/utf8_range C sources into the
		
    # google._upb._message extension - no protoc or Bazel is needed at this
		
    # stage, and no external native dependency (so, no CIBW_BEFORE_BUILD).

Normally we reserve comments for:

1. Patches to the source code in some way
2. Build config/metadata changes
3. Test skips

So I'm fine not to. What do you think @luhenry?

[luhenry]

It doesn't hurt to leave a note!