Mention in README that Catalina can't initialize the network drivers out of the box

[doughsay]

The only way we've found to get the Xebow's networking to work on macOS Catalina is to use https://github.com/jwise/HoRNDIS

And specifically, HoRNDIS also doesn't work out of the box on Cataline, so we had to use this package someone provided in the comments: jwise/HoRNDIS#102 (comment)

This should all be mentioned in the README.