Update README with basic WiFi 4/5/6 details

[meeas]

Looking through the README and the more extensive documentation at Nuand, it seems like this might be a full implementation of WiFi 4 (802.11n). I see the channel size seems to max out at 20 MHz and the modulation max out at OFDM-64-QAM. Does this mean this stack supports WiFi 5/6 as long as you are using that 20 MHz channels with (FDM-64-QAM? Any updates to the documentation to specify this would be great!

BTW, excellent work! Thank you for your efforts!