improve documentation

[matiasandina]

From a user, these are the comments:

When I start with a Rasberry Pi that already installed an OS, I click the jump here link then skip the raspi-config step and continue the installation without open the serial port. Then got a lot of error when running the GUI. Also I cannot find the GL driver and fake KMS option in my OS so it confused me a little bit.

• For the config file, I removed the first three lines (since use HAT) then has error /dev/ttyAMA4 is missing, but when using the full 6 lines, everything is fine. So maybe need to clarify how to edit the config file.
  ☝️ Updated README

• The demo FED3 sketch (ITI_03.INO) has a 15min timeout in default, maybe better to remove that.
  ☝️ Addressed in 11c8cdb

• include some guidance for basic setup will be helpful (e.g how to flash a image via Raspberry Pi Imager or install git)