README enhancement: Document k8sAppLabelOverride for kube-dns compatibility

[JohnYoungSuh]

README Enhancement: Document k8sAppLabelOverride for kube-dns Compatibility

Summary

The CoreDNS Helm chart supports k8sAppLabelOverride (added in PR #77) to fix label mismatches between CoreDNS pods and the legacy kube-dns service.

However, this setting is not documented in the README, and users deploying CoreDNS via Helm may encounter DNS failures unless they know to override the label.

Real-World Impact

This silent failure can break DNS resolution in new clusters, and users may not realize the root cause without deep inspection of service selectors and pod labels.

Without this documentation, users experience:

1. CoreDNS pods appear healthy (kubectl get pods shows Running)
2. Logs show zones loaded correctly
3. But DNS queries fail: nslookup: connection refused
4. Service endpoints remain empty: kubectl get endpoints kube-dns returns no IPs
5. Requires checking service selectors vs pod labels to diagnose

This cost our team 3+ hours of debugging time.

Suggested Documentation Addition

Location: README.md, Configuration section

Kubernetes Compatibility

If your cluster uses the legacy kube-dns service for DNS resolution, set:

k8sAppLabelOverride: "kube-dns"

[hagaibarel]

Thanks for raising this, indeed an important addition to the README.

I'll be happy to review a PR with the change