Add port-publishing use case

[l-dobrev]

Add a paragraph explaining how to use this image to publish ports on existing containers without recreating them.

[ozbillwang]

Can you change the line ### Getting Started to ### Example with consistent statement.

[l-dobrev]

How's that?

Also fixed a missing back-slash in the example.
Also changed upper-case arguments to lower-case to match the documentation docker run --rm alpine/socat -h.

[ozbillwang]

I can't make it work with --link example-container=target, I have replaced example-container name to a real one.

seems = can't be used with option --link, which docker version you have?

Here is mine:

$ docker -v
Docker version 18.03.1-ce, build 9ee9f40

[l-dobrev]

You are right!
The correct separator was ":" not "=".
I have no excuse for missing that. :(