add ref docs for node label

Signed-off-by: Charles Smith <charles.smith@docker.com>

docs/reference/commandline/node_update.md

@@ -25,6 +25,38 @@ Options:
       --role string           Role of the node (worker/manager)
 ```
 
+### Add label metadata to a node
+
+Add metadata to a swarm node using node labels. You can specify a node label as
+a key with an empty value:
+
+``` bash
+$ docker node update --label-add foo worker1
+```
+
+To add multiple labels to a node, pass the `--label-add` flag for each label:
+
+``` bash
+$ docker node update --label-add foo --label-add bar worker1
+```
+
+When you [create a service](service_create.md),
+you can use node labels as a constraint. A constraint limits the nodes where the
+scheduler deploys tasks for a service.
+
+For example, to add a `type` label to identify nodes where the scheduler should
+deploy message queue service tasks:
+
+``` bash
+$ docker node update --label-add type=queue worker1
+```
+
+The labels you set for nodes using `docker node update` apply only to the node
+entity within the swarm. Do not confuse them with the docker daemon labels for
+[dockerd]( ../../userguide/labels-custom-metadata.md#daemon-labels).
+
+For more information about labels, refer to [apply custom
+metadata](../../userguide/labels-custom-metadata.md).
 
 ## Related information