Add how to enable cAdvisor support on GKE

docs/GKE-cadvisor-support.md

+# Kubelet / cAdvisor special configuration updates for GKE 
+
+In order to allow Prometheus to access the endpoints provided by the kubelet/cAdvisor on GKE we have to downgrade the scheme to HTTP (from HTTPS).
+
+
+On linux:
+
+```
+sed -i -e 's/https/http/g' \
+contrib/kube-prometheus/manifests/prometheus/prometheus-k8s-service-monitor-kubelet.yaml
+```
+
+On MacOs: 
+
+``` 
+sed -i '' -e 's/https/http/g' \
+contrib/kube-prometheus/manifests/prometheus/prometheus-k8s-service-monitor-kubelet.yaml
+```
+
+After you have modified the yaml file please run
+
+```
+kubectl apply -f contrib/kube-prometheus/manifests/prometheus/prometheus-k8s-service-monitor-kubelet.yaml
+```