Add comments to gateway.yaml

Signed-off-by: Whitney Lee <[email protected]>

examples/istio/gateway.yaml

@@ -3,6 +3,8 @@ kind: Gateway
 metadata:
   name: istio-rollout-gateway
 spec:
+  # The selector matches the ingress gateway pod labels.
+  # If you installed Istio using Helm following the standard documentation, this would be "istio=ingress"
   selector:
     istio: ingressgateway
   servers: