adds pi finder build env setup instructions
adds pi finder build env setup instructions

octoblu/Adafruit-Pi-Finder,StealthMicro/Adafruit-Pi-Finder,octoblu/Adafruit-Pi-Finder,adafruit/Adafruit-Pi-Finder,CalmEQ/calmeq-pi-setup,StealthMicro/Adafruit-Pi-Finder,CalmEQ/calmeq-pi-setup,adafruit/Adafruit-Pi-Finder,CalmEQ/calmeq-pi-setup