Update the README.md to remind users that the correct way to use sql gateway with session cluster executors.

[V1ncentzzZ]

Caused by: java.lang.RuntimeException: Could not execute program. at com.ververica.flink.table.gateway.deployment.ProgramDeployer.deploy(ProgramDeployer.java:83) ~[flink-sql-gateway-0.2-SNAPSHOT.jar:?] at com.ververica.flink.table.gateway.operation.SelectOperation.executeQueryInternal(SelectOperation.java:247) ~[flink-sql-gateway-0.2-SNAPSHOT.jar:?] ... 46 more Caused by: java.lang.IllegalStateException at org.apache.flink.util.Preconditions.checkState(Preconditions.java:179) ~[flink-dist_2.12-1.11.1.jar:1.11.1] at org.apache.flink.client.deployment.executors.AbstractSessionClusterExecutor.execute(AbstractSessionClusterExecutor.java:61) ~[flink-dist_2.12-1.11.1.jar:1.11.1] at com.ververica.flink.table.gateway.deployment.ProgramDeployer.deploy(ProgramDeployer.java:81) ~[flink-sql-gateway-0.2-SNAPSHOT.jar:?] at com.ververica.flink.table.gateway.operation.SelectOperation.executeQueryInternal(SelectOperation.java:247) ~[flink-sql-gateway-0.2-SNAPSHOT.jar:?]

The program will thorw exception if we don't set yarn.application.id option in $FLINK_HOME/conf/flink-conf.yaml.