Documenting signal level displayed at the top of the menu

[acrostich]

I thought the RSSI displayed at the top of the menu was the RSSI of the connected device, but that's not the case. For instance currently the RSSI displayed at the top of the menu is -50 dBm while the one showing for the connected device in the Device submenu is -56 dBm. So what is the main menu RSSI of?

[ts1]

That's a 5-time moving average of observed RSSI. I'll fix README later.