Improve Cassandra sample README documentation

Update the README file in `spring-boot-sample-data-cassandra` with
details of how to setup the keyspace and create the tables.

Closes gh-7737

spring-boot-samples/spring-boot-sample-data-cassandra/README.adoc

+= Spring Boot Sample Data Cassandra
 
+To run the project, need to run below `cql` commands on Cassandra.
 
-CREATE KEYSPACE mykeyspace WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };
+== Keyspace Creation in Cassandra
+[source,indent=0]
+----
+	CREATE KEYSPACE mykeyspace WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };
+----
 
+== Table Creation in Cassandra
+Run `cql` using the  link:src/test/resources/setup.cql[setup script] located in resources folder.