Add docker run -p to README (fixes #5)

[vorburger]

see #9 (not #5)

@yegle OK for you?

[yegle]

I think I'm more comfortable having a different section asking the user to add -p 5000:5000 if they want to expose the service directly.

Something like this:

NOTE: The command above does not expose service directly, so you can use your own reverse proxy. If you do want to expose the service without using a reverse proxy, you can add `-p 5000:5000` and access the service at the corresponding port.

[vorburger]

@yegle I see what you mean. How about like this?

[yegle]

Thanks for the contribution!