Add warning to Dockerfile re: upgrading kubectl

cmd/kured/Dockerfile

 FROM ubuntu:16.04
 RUN apt-get update && apt-get install -y ca-certificates && rm -rf /var/cache/apt
+# NB: you may need to update RBAC permissions when upgrading kubectl - see kured-rbac.yaml for details
 ADD https://storage.googleapis.com/kubernetes-release/release/v1.10.3/bin/linux/amd64/kubectl /usr/bin/kubectl
 RUN chmod 0755 /usr/bin/kubectl
 COPY ./kured /usr/bin/kured