suggested configuration results in open DNS resolver

[soletan]

The configuration suggested for this plugin is turning a CoreDNS sservice into an open DNS resolver when using CoreDNS as a regular DNS server (not hidden in a k8s cluster). Having a warning as a minimum precaution on using that configuration would be helpful. Having a more specific configuration example limiting use of forward plugin to a certain domain would be even better, though I'm not 100% sure how secure this would be eventually.