Add an example for --with-registry-auth

Signed-off-by: Misty Stanley-Jones <misty@docker.com>

docs/reference/commandline/service_create.md

@@ -112,6 +112,26 @@ dmu1ept4cxcf  redis   replicated  1/1       redis:3.0.6
 a8q9dasaafud  redis2  global      1/1       redis:3.0.6
 ```
 
+#### Create a service using an image on a private registry
+
+If your image is available on a private registry which requires login, use the
+`--with-registry-auth` flag with `docker service create`, after logging in. If
+your image is stored on `registry.example.com`, which is a private registry, use
+a command like the following:
+
+```bash
+$ docker login registry.example.com
+
+$ docker service  create \
+  --with-registry-auth \
+  --name my_service \
+  registry.example.com/acme/my_image:latest
+```
+
+This passes the login token from your local client to the swarm nodes where the
+service is deployed, using the encrypted WAL logs. With this information, the
+nodes are able to log into the registry and pull the image.
+
 ### Create a service with 5 replica tasks (--replicas)
 
 Use the `--replicas` flag to set the number of replica tasks for a replicated