docs(contributing): add a contributor guide

CONTRIBUTING.md

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+# Contributing
+
+`rig-c/getting-started` is the step-by-step guide that takes a newcomer from one character painting to a rigged character breathing, blinking, and waving in the browser, together with the scripts that test every step of the walkthrough.
+
+## Getting the code running
+
+You need [Bun](https://bun.sh) 1.4+ (and [uv](https://docs.astral.sh/uv/) if you run the Python keypoint script in step 6). The *Try it first* walkthrough in [README.md](README.md#try-it-first--no-gpu-about-3-minutes) runs in a separate working directory outside this repository using pre-built example inputs and requires no GPU. Only the optional painting and See-through layer-extraction steps of *Make your own character* use a GPU or the See-through web demo.
+
+## Before you open a pull request
+
+Run the fast offline README, link-reader, and walkthrough-extractor checks (from `.github/workflows/adapters.yml` and [AGENTS.md](AGENTS.md)):
+
+```sh
+bun scripts/check-readme.ts
+bun test scripts/check-readme.test.ts
+bun test scripts/link-reader.test.ts
+bun test scripts/try-it.test.ts
+```
+
+When a change touches walkthrough commands, helper scripts, or adapter snippets, also run the end-to-end browser and walkthrough checks (long runs that install published packages from npm, download inputs, and launch headless browsers; `try-it.ts` requires `TRY_IT_TEMP_ROOT` set to a directory outside the checkout):
+
+```sh
+bun scripts/check-adapters.ts
+TRY_IT_TEMP_ROOT=<temp-dir-outside-repo> bun scripts/try-it.ts --mode force --out <out-dir>
+```
+
+In CI, `.github/workflows/adapters.yml` runs `bun scripts/check-adapters.ts --install-system-deps` and `bun scripts/try-it.ts --mode "$TRY_IT_MODE"`.
+
+## Reporting a change
+
+- **Issue first.** Open an issue before changing the guide or its scripts, naming the step, the command you ran, its verbatim output, and what you expected.
+- **One issue per pull request.** Keep each pull request focused on a single issue.
+- **Conventional Commits.** Write commit messages and documentation in English using [Conventional Commits](https://www.conventionalcommits.org/) with a scope (`fix(guide):`, `test(try-it):`, `docs(readme):`). Keep all tracked files free of private names and local paths.
+
+## Where further rules live
+
+- [AGENTS.md](AGENTS.md) — README scope, word-count cap, heading allowlist, and link rules.
+- [docs/REFERENCE.md](docs/REFERENCE.md) — tested package records, environment details, and detailed measurements.
+- [README.md](README.md) and [LICENSE](LICENSE) — the walkthrough and MIT licence terms.