docs: usb_gadget_path for Android

[anonhostpi]

On my Samsung Galaxy S8+ Snapdragon, the usb_gadget_path is '/config/usb_gadget/', and not '/sys/kernel/config/usb_gadget/'

As Android is likely a key target platform for this library (because of hardware availability), it's probably worth mentioning the different path in the README.

Also worth noting that python must be run elevated on Android to use this library.