README: add versioning disclaimer, link to samples
README: add versioning disclaimer, link to samples

aliafshar/firebase-arduino,aliafshar/firebase-arduino,gguuss/firebase-arduino,gguuss/firebase-arduino,FirebaseExtended/firebase-arduino,gguuss/firebase-arduino,aliafshar/firebase-arduino,FirebaseExtended/firebase-arduino,FirebaseExtended/firebase-arduino