nit: Add offline note for kube-* images (#8718)

contrib/offline/generate_list.sh

@@ -19,6 +19,9 @@ sed -n '/^downloads:/,/download_defaults:/p' ${REPO_ROOT_DIR}/${DOWNLOAD_YML} \
     | sed 'N;s#\n# #g' | tr ' ' ':' | sed 's/\"//g' > ${TEMP_DIR}/images.list.template
 
 # add kube-* images to images list template
+# Those container images are downloaded by kubeadm, then roles/download/defaults/main.yml
+# doesn't contain those images. That is reason why here needs to put those images into the
+# list separately.
 KUBE_IMAGES="kube-apiserver kube-controller-manager kube-scheduler kube-proxy"
 for i in $KUBE_IMAGES; do
     echo "{{ kube_image_repo }}/$i:{{ kube_version }}" >> ${TEMP_DIR}/images.list.template