Documentation suggestions

[invd]

• Currently the README.md does not explain what J3 and SW3 are for. This is only mentioned in the introduction video. The silkscreen around SW3 says HOLD 5S DISCHARGE but that could be misinterpreted as regular discharge into the coil.
• Q1, D8 and D9 have no easily visible orientation mark on the printed silkscreen. The build-drawing PDF helps a bit.
• It is not clear if the device is allowed to have voltage on J2 at while the USB port is in use and how it will behave in terms of power draw. Based on a short look at the Pico2040 datasheet on page 19, dual operation is acceptable and the USB power supply will become the primary due to its higher supply voltage after the Schottky diode compared to the 2xAA source + Schottky diode. However, the datasheet notes that USB is required if the pico firmware uses USB host mode, which may be the case with the C firmware version.