docs: add buffa-build README

[fallintoplace]

What

• Add a README for buffa-build.

Why

• The crate page had no quick setup or compiler requirements.

Implementation

• Show the dependencies, a build.rs example, and the compiler choices.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[iainmcgin]

[claude code] Thanks for writing a README for buffa-build, which had none on crates.io. I pushed two maintainer commits on top; the examples are unchanged. The first replaces the dependency snippet that pinned "0.9" three times with the cargo add commands from the user guide, so the page stays right after the next release, and removes the changelog fragment, because a README-only change does not get a changelog entry in this repository. The second moves the "Protobuf compiler" section above the quick start, since the example needs the compiler, and adds the minimum protoc versions from the user guide. It says what .use_buf() needs (a buf.yaml at the crate root; .includes() is ignored; .files() paths are relative to the buf module directory), points the compiler link at the prerequisites section of the guide, and marks buffa-types as optional in the command block.