update stack deploy instruction

README.md

@@ -28,6 +28,16 @@ Clone the project locally to your Docker host.
 
 If you would like to change which targets should be monitored or make configuration changes edit the [/prometheus/prometheus.yml](https://github.com/vegasbrianc/prometheus/blob/version-2/prometheus/prometheus.yml) file. The targets section is where you define what should be monitored by Prometheus. The names defined in this file are actually sourced from the service name in the docker-compose file. If you wish to change names of the services you can add the "container_name" parameter in the `docker-compose.yml` file.
 
+**Note**: before deploy the stack, need modify the docker-compose.yml
+```
+...
+deploy:		
+  placement:
+    constraints:
+      - node.hostname == <hostname-of-swarm-node-where-config-files-exist>
+...
+```
+
 Once configurations are done let's start it up. From the /prometheus project directory run the following command:
 
     $ docker stack deploy -c docker-compose.yml prom