[Goldloli]

What

Adds CONTRIBUTING.md, the how-to document requested in #284 ("The project misses a simple how-to document explaining how to contribute and work with the source code"). It covers:

• Requirements & setup — Node.js >= 18 (from engines), npm install, and npm run test:setup (Firefox/WebKit via Playwright) plus a local Chrome for the headless Chrome run.
• Repository layout — src/ (plain ES modules, no build step), test/spec/, test/types/, types/, cli/, docs/, and the CI workflow.
• Commands — npm run dev (Karma watch in Chrome headless), npm run lint, npm run format, npm run test:cli, npm run test:typescript, npm run karma, npm run test:coverage, and what the full npm test runs.
• Documentation — docsify site in docs/, navigation in _sidebar.md / _navbar.md, how to add a page; note that CHANGELOG.md is generated by standard-version.
• Commit convention — Conventional Commits with examples from the repository history.
• Pull request flow — branch from main, focused change + tests, run npm test, the CI Test workflow steps (npm install → npm run test:setup → npm test).
• Release process — maintainers only: npm run release.

Why

The repository has no contributing documentation, and #284 tracks exactly that gap. Without it, new contributors have to reverse-engineer the Karma/Playwright setup (test:setup) and the individual checks from .github/workflows/test.yml.

Verification

Every command documented in the guide was executed locally on this branch:

Command	Result
npm install	468 packages installed, exit 0
npm run test:cli	pass 5, fail 0
npm run lint	clean (no output)
npm run test:typescript	clean (no output)
npm run karma -- --browsers ChromeHeadless	616 of 616 SUCCESS (47.9 s)

npm run test:setup (Playwright browser download) and the full three-browser npm test were not run locally; CI runs them exactly as documented. The Node.js 18+ requirement is taken from engines in package.json (CI uses Node 22).

Closes #284

[smalluban]

The document is correct. @Goldloli thanks for the contribution.

[coveralls]

coverage: 99.875%. remained the same — Goldloli:contributing-guide into hybridsjs:main