Add a brief note about Docker Swarm

A deeper comparison would be nice at some point.

README.md

@@ -43,12 +43,14 @@ This will:
 
 Voila! All the servers are now serving the app on port 80. If you're just running a single server, you're ready to go. If you're running multiple servers, you need to put a load balancer in front of them.
 
-## Why not just run Capistrano or Kubernetes?
+## Why not just run Capistrano, Kubernetes or Docker Swarm?
 
 MRSK basically is Capistrano for Containers, which allow us to use vanilla servers as the hosts. No need to ensure that the servers have just the right version of Ruby or other dependencies you need. That all lives in the Docker image now. You can boot a brand new Ubuntu (or whatever) server, add it to the deploy servers of MRSK, and it'll be auto-provisioned with Docker, and run right away. Docker's layer caching also allows for quicker deployments with less mucking about on the server. And the images built for MRSK can be used for CI or later introspection.
 
 Kubernetes is a beast. Running it yourself on your own hardware is not for the faint of heart. It's a fine option if you want to run on someone else's platform, like Render or Fly, but if you'd like the freedom to move between cloud and your own hardware, or even mix the two, MRSK is much simpler. You can see everything that's going on, it's just basic Docker commands being called.
 
+Docker Swarm is much simpler than Kubernetes, but it's still built on the same declarative model that uses state reconciliation. MRSK is intentionally designed to around imperative commands, like Capistrano.
+
 ## Configuration
 
 ### Using .env file to load required environment variables