Update README for create-cluster

[yyoshiki41]

Add call command usage in README.

$ ./create-cluster
Usage: ./create-cluster [start|create|stop|watch|tail|call|clean]
start       -- Launch Redis Cluster instances.
create      -- Create a cluster using redis-trib create.
stop        -- Stop Redis Cluster instances.
watch       -- Show CLUSTER NODES output (first 30 lines) of first node.
tail <id>   -- Run tail -f of instance at base port + ID.
call        -- Run the given command to each nodes.
clean       -- Remove all instances data, logs, configs.

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This change is

[gkorland]

not relevant any more