I added CONTRIBUTING.md at the repo root. It has three sections, Build, Test and Propose a change, in the same style as the README (ATX headings, sh code blocks). No other files changed.
I ran cargo build, cargo run -- --shout Ada (prints HELLO, ADA!) and cargo test (all tests pass), and the file exists.
The issue asks contributors to add an entry under Unreleased in CHANGELOG.md, and the guide says so. But the repo has no CHANGELOG.md, and the issue said not to touch other files, so I didn't create one. Until someone adds it, that step in the guide points at a file that isn't there. Either add a changelog in a follow-up or drop the step.
I did not check the fork, branch and pull request steps against any hosting setup.