Add CONTRIBUTING.md explaining the linting setup and how to mark the
repo's intentional dummy credentials so gitleaks does not flag them:
inline `# gitleaks:allow` for lint-clean, comment-capable files, and a
.gitleaksignore baseline otherwise.
Annotate the dummy credentials in four already-lint-clean example files
inline, as the preferred convention going forward.
- DIUN/docker-compose.yaml
- Headscale/Tailscale-Client/docker-compose,yaml
- Homepage/Homepage/services.yaml
- Kubernetes/Traefik-PiHole/Helm/Traefik/Dashboard/secret-dashboard.yaml
