feat: add contributing

CONTRIBUTING.md

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+# Contributing
+
+Thanks for your interest in contributing to var-th!
+
+## Getting started
+
+```bash
+git clone https://github.com/kirilinsky/varth
+cd varth
+npm install
+```
+
+## Development
+
+```bash
+npm run dev           # watch mode
+npm test              # run tests
+npm run build         # build
+npm run test:coverage # check coverage
+```
+
+## Before opening a PR
+
+- tests pass — `npm test`
+- types check — `npm run typecheck`
+- new functionality has tests — coverage is tracked via Codecov and PRs without tests will not be merged
+
+## Coverage
+
+We use [Codecov](https://codecov.io/github/kirilinsky/varth) to track test coverage. Every new feature or bug fix must come with tests. Check your coverage locally before opening a PR:
+
+```bash
+npm run test:coverage
+```
+
+## Project structure
+
+```
+src/
+├── index.ts          # core — defineThemes()
+├── types.ts          # shared types
+├── constants/        # constants
+└── react/            # React provider + useTheme
+```
+
+## Adding a new framework
+
+React implementation in `src/react/` can serve as a reference.
+New framework entrypoints go in `src/{framework}/index.tsx`. Looking for help with Vue and Svelte integrations.
+Don't forget to add the entrypoint to `tsdown.config.ts` and `exports` in `package.json`.
+
+## Issues & discussion
+
+Found a bug or have an idea — open an issue first before starting a big PR.