docs: add CONTRIBUTING.md

Author: ronaldtse

CONTRIBUTING.md

@@ -0,0 +1,42 @@
+# Contributing
+
+Thanks for your interest in contributing!
+
+## Development setup
+
+```bash
+git clone <this repo>
+cd <repo>
+bundle install         # Ruby projects
+# or
+npm ci                 # JS projects
+```
+
+## Workflow
+
+1. Fork → branch from `main`
+2. Make changes with tests
+3. Run `bundle exec rspec` (Ruby) or `npm test` (JS) locally
+4. Run `bundle exec standardrb` (Ruby) or `npm run lint` (JS)
+5. Open a PR with a clear description
+
+## Code style
+
+- Ruby: enforced by [StandardRB](https://github.com/standardrb/standard)
+- JavaScript: enforced by ESLint + Prettier
+- Python: enforced by ruff
+
+## Commit messages
+
+Use [Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+feat: add new transliteration system
+fix: correct off-by-one in CALT lookup
+chore: bump dependencies
+docs: clarify README
+```
+
+## Releases
+
+Maintainers tag releases following semver. CI publishes on tag push.