update ReadMe.md section "Building from Source"

[mi-hol]

Is your feature request related to a problem? Please describe. sync doc with enhancement made in https://github.com/espotek-org/Labrador/commit/4e71cc9e6e4a5edaa32ad160f77edd33a69e1eef

Describe the solution you'd like reference new Makefile AVR_Code/USB_BULK_TEST/Makefile

@brentfpage may I kindly ask you to own this task?