Add networking section to README.md

[straysheep-dev]

This adds a section for network considerations and guidance on disabling unnecessary services that may be bound to all interfaces (and in some cases are listening by default). While the risk is relatively low here, it's always a good idea to disable services that aren't needed or in use.

I've frequently repeated these steps / checks when, for example, running a live OS to download and update firmware over a flat or untrusted network. I didn't see any direct mentions to this in the issues or past pull requests, but it seemed like it could fit with the rest of the guide.