Add CONTRIBUTING.md explaining how to build, test and propose a change #71

Create CONTRIBUTING.md at the repo root for outside contributors. The crate is hello (Rust 2021 edition, see Cargo.toml), with logic in src/lib.rs (parse_args), entry point src/main.rs, and integration tests in tests/cli.rs.

Cover:

• Build: cargo build, and running with cargo run -- --shout Ada.
• Test: cargo test (the README's Develop section uses it).
• Propose a change: fork, branch, make a focused change with tests, make sure cargo test passes, update README usage when flags change, add an entry under Unreleased in CHANGELOG.md, open a pull request describing what and why.

Only state commands that actually work; verify them by running them. Do not modify other files. Keep formatting consistent with README.md (ATX headings, fenced sh code blocks).

Definition of done

• cargo build passes.
• cargo test passes.
• test -f CONTRIBUTING.md passes.