Added comparison with stackdriver-prometheus
Added comparison with stackdriver-prometheus

The motivation is that the Stackdriver support team gets user requests to support prometheus-to-sd, even though we have an alternative that some of these users aren't aware of.
kawych/k8s-stackdriver,kawych/k8s-stackdriver,kawych/k8s-stackdriver,GoogleCloudPlatform/k8s-stackdriver,kawych/k8s-stackdriver,GoogleCloudPlatform/k8s-stackdriver,GoogleCloudPlatform/k8s-stackdriver