Document used ports

Sorry for the mess I made, but as I'm working on setting up a cluster, I was trying to figure out the firewall rules I'd need. And I'm having a hard time figuring out which port/protocol is needed for what.

I like to only open what's really needed, and this PR is also an attempt to get feedback from you, to make sure I understood ;)

I also have trouble understanding the difference between `8300` and `8400`.

Feel free to rephrase/correct/dump.

Thanks!