Add BoM and basic wiring guide to README

[proffalken]

At present, anyone putting together one of these devices needs to read through the code to understand the hardware required and how it might be wired up.

As an example, config.py simply states there is a pin for a buzzer, but there is no indication as to the type of buzzer that is required or whether it should be wired with +ve to the pin or -ve to the pin.

It would be good to get a BoM for the components used, and a basic table along the lines of the following:

Device Name	Config Pin	Device Pin
Buzzer	BUZZER_PIN	+ve
	GND	-ve
Led	LED_PIN	+ve
	GND	-ve
RDM6300	TX	RX
	RX	TX