Add some installation hints for Raspberry Pi (esp. Zero W)

[Michael Huebler]

When I tried to install btlejack on a Raspberry Pi Zero W, I followed the instructions from this Blog post: SSH into Pi Zero over USB.
This enables virtual ethernet over USB and allows to initially connect to the Pi Zero W, to set it up and install stuff like btlejack.
In order to use btlejack, I needed the USB port for the sniffers, and therefore configured the Pi Zero W to connect to WLAN, but I forgot to disable the virtual ethernet driver, i.e. I didn't remove dtoverlay=dwc2 and modules-load=dwc2,g_ether.
This basically broke btlejack completely, because this driver tried to use the sniffer USB connection, and all kinds of strange issues occurred: The Pi Zero W became completely unresponsive on ssh; when I tried this on a Pi 3 B+, the usb devices /dev/ttyACM0, /dev/ttyACM1, ... were blocked and couldn't be accessed by btlejack; and many more strange issues.

So I suggest adding a hint for Raspberry Pi users:

Hint for using btlejack on a Raspberry Pi:

If you have previously enabled virtual ethernet over USB (RNDIS), e.g. to setup a Raspberry Pi Zero W over USB, you need to disable this again (i.e. remove dtoverlay=dwc2 from boot/config.txt and modules-load=dwc2,g_ether from boot/cmdline.txt, then sudo reboot), because this would otherwise interfere with the sniffers' USB connections.

[Damien Cauquil]

You are absolutely right, this could help some users trying to make it work on a Raspberry Pi. I did not try it myself, so I guess it will be improved by contributors (me included when I will get my hands on a Raspberry Pi ...).

Can you do a pull request with the modified readme file ? Or if you do not want to, I will modify it accordingly.

[Damien Cauquil]

Allright, thank you again for your contribution !