docs: add contributing.md

Author: VladislavKudrin

CONTRIBUTING.md

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+# Contributing
+
+## Maintainer
+
+This project is maintained by the [Cardano Foundation](https://cardanofoundation.org).
+Current maintainer: [@VladislavKudrin](https://github.com/VladislavKudrin)
+
+## Reporting Issues
+
+Open a [GitHub issue](https://github.com/cardano-foundation/cardano-graphql/issues) with:
+- Steps to reproduce
+- Expected vs actual behavior
+- Environment (OS, Docker version, cardano-node/db-sync versions)
+
+## Submitting Pull Requests
+
+1. Fork the repository
+2. Create a branch from `master`
+3. Make your changes
+4. Ensure lint passes: `yarn workspaces run lint`
+5. Open a PR against `master`
+
+## Running Locally
+
+See [README.md](README.md) for setup instructions.
+
+To run tests against a live preprod node:
+```bash
+GRAPHQL_URL=http://localhost:<port> yarn workspace @cardano-graphql/api-cardano-db-hasura test --forceExit
+```
+GraphQL golden tests (requires a running mainnet node):
+```bash
+cd tests
+uv run pytest
+```
+
+## Code of Conduct
+
+Please follow our [Code of Conduct](CODE-OF-CONDUCT.md).