add network isolation instructions

[99z]

Hi, thanks for your work on this project. It's really the best experience I've had with local LLMs on my Framework Desktop.

I know some people are nervous about the project being closed-source for the time being, which I think is reasonable. For people on the fence about running Halogen, I thought it might be helpful to include instructions to deny outbound network traffic from the container. At least they can be confident nothing malicious (at least via network) is happening :)

I'm not sure if README PRs are accepted so feel free to copy/paste the contents or write your own instructions. Or ignore this completely.

Cheers!

[suvayu]

I will also add, that turning up your firewall, and running something like tailscale, and then connecting only over that network is pretty simple to setup and maintain without exposing yourself to any external risks.