feat: add CONTRIBUTING.md

Author: grishy

CONTRIBUTING.md

@@ -0,0 +1,80 @@
+## Development Setup
+
+```bash
+git clone https://github.com/grishy/go-avahi-cname.git
+cd go-avahi-cname
+go mod download
+go build -o go-avahi-cname .
+```
+
+## Before Committing
+
+Run formatting, linting, and tests:
+
+```bash
+golangci-lint fmt ./...
+golangci-lint run ./...
+go test -race -shuffle=on -vet=all -failfast ./...
+```
+
+All three should pass with no errors.
+
+## Testing Locally
+
+You need a Linux machine with Avahi daemon running.
+
+```bash
+# Run with debug logging
+./go-avahi-cname --debug subdomain
+
+# From another terminal or device
+ping anything.yourhostname.local
+```
+
+## Code Style
+
+This project uses [golangci-lint](https://golangci-lint.run/) with a strict
+config (see `.golangci.yml`). The linter handles formatting via `goimports`,
+`gofumpt`, and `golines`.
+
+A few principles:
+
+- Meaningful variable names — no abbreviations unless standard
+- Comments explain "why," not "what"
+- Keep functions focused and reasonably sized
+
+## Submitting Changes
+
+1. Fork the repository
+2. Create a feature branch: `git checkout -b feature/your-feature`
+3. Make changes, run the checks above
+4. Push to your fork and open a Pull Request
+
+## Reporting Issues
+
+Please include:
+
+- OS and version
+- Avahi daemon version (`avahi-daemon --version`)
+- Output with `--debug` flag
+- Steps to reproduce
+
+## Release Process (Maintainers)
+
+1. Update version references in the project.
+
+2. Create and push a tag:
+
+   ```bash
+   git tag -a v2.5.0 -m "Release v2.5.0"
+   git push origin tag v2.5.0
+   ```
+
+3. GitHub Actions will:
+   - Build binaries for all platforms
+   - Create a GitHub release
+   - Push Docker images to GHCR
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the MIT License.