Documentation: better description of the network testing procedure for sim

For the `sim:nettest` target it fails. This could very easily be an issue not configuring the environment correctly, but there is no documentation in the readme about what needs to be setup if anything. Is probably does not need to be a blocker, but we need to document how this works

```
❯ ./nuttx
[sudo] password for bashton:
Connecting to Address: 0100000a
client: connect failure: 114
```