cmd/add: clarify "add" is only for configmap
cmd/add: clarify "add" is only for configmap

Signed-off-by: Gyuho Lee <0fbd30d364d71722b926950b451cecd97b54fc29@amazon.com>

kubernetes-sigs/aws-iam-authenticator,kubernetes-sigs/aws-iam-authenticator,kubernetes-sigs/aws-iam-authenticator