Mention use of sudo with swarm

For the Docker Swarm deployment guide we should mention the use
of sudo for Linux users who do not want to give their accounts
access to the docker group. [0]

[0] openfaas/faas#664

Signed-off-by: Alex Ellis (VMware) <[email protected]>

Author: alexellis

docs/deployment/docker-swarm.md

@@ -1,5 +1,7 @@
 # Deployment guide for Docker Swarm
 
+> Note: If you need to use `sudo` to access the `docker` CLI then you should prefix this behind any shell scripts or CLIs used in this guide or any related tutorials.
+
 ## 1.0 Initialize Swarm Mode
 
 You can create a single-host Docker Swarm on your laptop with a single command. You don't need any additional software to Docker 17.06 or greater. You can also run these commands on a Linux VM or cloud host.