Add docs for service create based on plugins.

Signed-off-by: Anusha Ragunathan <anusha.ragunathan@docker.com>
(cherry picked from commit 62d399e8112caacad93829d8faa1c15dfc4f694c)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

docs/extend/index.md

@@ -107,6 +107,62 @@ remove it, use the `docker plugin remove` command. For other available
 commands and options, see the
 [command line reference](../reference/commandline/index.md).
 
+## Service creation using plugins
+
+In swarm mode, it is possible to create a service that allows for attaching
+to networks or mounting volumes. Swarm schedules services based on plugin availability
+on a node. In this example, a volume plugin is installed on a swarm worker and a volume 
+is created using the plugin. In the manager, a service is created with the relevant
+mount options. It can be observed that the service is scheduled to run on the worker
+node with the said volume plugin and volume. 
+
+In the following example, node1 is the manager and node2 is the worker.
+
+1.  Prepare manager. In node 1:
+
+    ```bash
+    $ docker swarm init
+    Swarm initialized: current node (dxn1zf6l61qsb1josjja83ngz) is now a manager.
+    ```
+
+2. Join swarm, install plugin and create volume on worker. In node 2:
+
+    ```bash
+    $ docker swarm join \
+    --token SWMTKN-1-49nj1cmql0jkz5s954yi3oex3nedyz0fb0xx14ie39trti4wxv-8vxv8rssmk743ojnwacrr2e7c \
+    192.168.99.100:2377
+    ```
+
+    ```bash
+    $ docker plugin install tiborvass/sample-volume-plugin
+    latest: Pulling from tiborvass/sample-volume-plugin
+    eb9c16fbdc53: Download complete
+    Digest: sha256:00b42de88f3a3e0342e7b35fa62394b0a9ceb54d37f4c50be5d3167899994639
+    Status: Downloaded newer image for tiborvass/sample-volume-plugin:latest
+    Installed plugin tiborvass/sample-volume-plugin
+    ```
+	
+    ```bash
+    $ docker volume create -d tiborvass/sample-volume-plugin --name pluginVol
+    ```
+
+3. Create a service using the plugin and volume. In node1:
+
+    ```bash
+    $ docker service create --name my-service --mount type=volume,volume-driver=tiborvass/sample-volume-plugin,source=pluginVol,destination=/tmp busybox top
+
+    $ docker service ls
+    z1sj8bb8jnfn  my-service   replicated  1/1       busybox:latest 
+    ```
+    docker service ls shows service 1 instance of service running.
+
+4. Observe the task getting scheduled in node 2:
+
+    ```bash
+    $ docker ps --format '{{.ID}}\t {{.Status}} {{.Names}} {{.Command}}' 
+    83fc1e842599     Up 2 days my-service.1.9jn59qzn7nbc3m0zt1hij12xs "top"
+    ```
+
 ## Developing a plugin
 
 #### The rootfs directory