Improve README first-play experience

In the course of experimenting with swarm I was confused by the README, and suggest the following improvements:

1. Use the official Docker port of 2375 everywhere.
2. Highlight that the `--addr` argument requires the public IP of the node to advertise.
3. Do not imply running multiple `swarm join` commands on each node.

I am not happy with `public_node_ip`, as it is not necessarily a public IP, just the node's addressable IP. Suggestions are solicited.