Update compose example to work better with swarm

Update the compose example:

* Remove the `links` directive. This allows swarm to distribute containers across available nodes. (with the `links` directive, swarm will be forced to run both containers on the same node.) Note that the `depends_on` directive is sufficient to ensure correct container start up sequence.
* Use a docker volume instead of bind mounting a local directory.
* Give docker-compose commands for shutting down the Wordpress app, showing how to preserve or remove the persisted data in the volume.

Verified proposed change works correctly in both a swarm and a non-swarm environment.

compose/wordpress.md

@@ -33,7 +33,7 @@ with Docker containers. This quick-start guide demonstrates how to use Compose t
        db:
          image: mysql:5.7
          volumes:
-           - "./.data/db:/var/lib/mysql"
+           - db_data:/var/lib/mysql
          restart: always
          environment:
            MYSQL_ROOT_PASSWORD: wordpress
@@ -45,19 +45,17 @@ with Docker containers. This quick-start guide demonstrates how to use Compose t
          depends_on:
            - db
          image: wordpress:latest
-         links:
-           - db
          ports:
            - "8000:80"
          restart: always
          environment:
            WORDPRESS_DB_HOST: db:3306
            WORDPRESS_DB_PASSWORD: wordpress
+    volumes:
+        db_data:
     ```
 
-    **NOTE**: The folder `./.data/db` will be automatically created in the project directory
-    alongside the `docker-compose.yml` which will persist any updates made by wordpress to the
-    database.
+    **NOTE**: The docker volume `db_data` will persist any updates made by wordpress to the database. [Learn more about docker volumes](../engine/tutorials/dockervolumes.md)
 
 ### Build the project
 
@@ -97,6 +95,10 @@ At this point, WordPress should be running on port `8000` of your Docker Host, a
 
 ![WordPress Welcome](images/wordpress-welcome.png)
 
+### Shutdown/Clean up
+`docker-compose down` will remove the containers and default network, but preserve your wordpress database.
+`docker-compose down --volumes` will remove the containers, default network, and the wordpress database.
+
 ## More Compose documentation
 
 - [User guide](index.md)
@@ -105,4 +107,4 @@ At this point, WordPress should be running on port `8000` of your Docker Host, a
 - [Get started with Django](django.md)
 - [Get started with Rails](rails.md)
 - [Command line reference](./reference/index.md)
-- [Compose file reference](compose-file.md)
+- [Compose file reference](compose-file.md)