added readme to sf vehicles example
added readme to sf vehicles example

firebase/geofire-java,marcosddourado/geofire-java,CASPED/geofire-java,marcosddourado/geofire-java,jgarcia162/geofire-java,marcosddourado/geofire-java,joreilly/geofire-java,joreilly/geofire-java,joreilly/geofire-java,CASPED/geofire-java,jgarcia162/geofire-java,firebase/geofire-java,firebase/geofire-java,CASPED/geofire-java,jgarcia162/geofire-java