Documentation request: add documentation on building the raspberry pi with the ultimate GPS hat

The repository currently contains scripts needed to extract NMEA message from the GNSS chip.

The function of this repository could be improved to also incorporate a guide to build the GNNSS-R raspberry itself. For this it would be good to: (1) Extend the top README.md as a "getting started guide" (2) Add a subfolder with hardware building instructions (pictures, readme files etc)