README.md:  fix a couple typos, add beta warning just-in-case
README.md:  fix a couple typos, add beta warning just-in-case

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