update README.md to make single-node/multi-node a little more clear
update README.md to make single-node/multi-node a little more clear

unusedPhD/cassandra-docker,tobert/cassandra-docker,pofallon/cassandra-docker,gitinsky/cassandra-docker-stat,unusedPhD/cassandra-docker,tobert/cassandra-docker,vitosans/cassandra-docker,mukadder/cassandra-docker,mukadder/cassandra-docker,vitosans/cassandra-docker,Ajaylko90/Cassandra,pofallon/cassandra-docker,kryzna/cassandra-docker,kryzna/cassandra-docker,Ajaylko90/Cassandra