docs: add contribution guidelines for Base node

docs/CONTRIBUTING.md

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+# Contributing to the Base node repository
+
+Thank you for considering a contribution!
+
+## Getting started
+
+1. Fork the repository and clone your fork.
+2. Run `docker compose up` to start a node locally.
+3. Use a new branch per change (`git checkout -b feature/your-description`).
+
+## Code style
+
+- Shell scripts should use `#!/usr/bin/env bash` and `set -euo pipefail`.
+- Go code follows `gofmt` style; run `go fmt ./...` before committing.
+- Document any new files or APIs.
+
+## Testing your changes
+
+- If you modify the compose setup, run `docker compose config` to ensure the YAML is valid.
+- For Go changes, run `go test ./...`.
+- For docs, ensure Markdown renders properly and links are correct.
+
+## Opening a pull request
+
+Describe:
+
+- The purpose of your change.
+- How you tested it.
+- Any related issues it addresses.
+
+Maintainers will review and provide feedback. Welcome aboard!