Added upgrade instructions to README
Added upgrade instructions to README

ObjSal/geofire-objc,firebase/geofire-objc,ObjSal/geofire-objc,firebase/geofire-objc,victoraliss0n/geofire-objc,victoraliss0n/geofire-objc,ObjSal/geofire-objc,firebase/geofire-objc,firebase/geofire-objc,victoraliss0n/geofire-objc