Document the packets required to check if UDP port is open

[pojntfx]

First of all: Thanks for the library! I'm currently working on building an efficient port scanner with it (to be open source soon) and it's of great use. While doing so I was wondering what the packets were for registered ports to get their open state (like nmap does), so I've build a tool that does that:

https://pojntfx.github.io/ports2packets/

It might be useful to put a link in the README or something for others to get started as well. I'd be happy to create a PR if you'd like me too ;)