Minor cleanup in samples and tools

- Cleanup in schema-registry samples (final variable).
- Add instructions in tools/kafka/docker-compose to run
  a quick test using Kafka console scripts.

samples/spring-cloud-stream-schema-registry-integration/schema-registry-producer1-kafka/src/main/java/sample/producer1/Producer1ApplicationKafka.java

@@ -35,7 +35,7 @@ import org.springframework.web.bind.annotation.RestController;
 @RestController
 public class Producer1ApplicationKafka {
 
-	private Random random = new Random();
+	private final Random random = new Random();
 
 	@Autowired
 	private StreamBridge streamBridge;

samples/spring-cloud-stream-schema-registry-integration/schema-registry-producer2-kafka/src/main/java/sample/producer2/Producer2ApplicationKafka.java

@@ -41,7 +41,7 @@ import com.example.Sensor;
 @RestController
 public class Producer2ApplicationKafka {
 
-	private Random random = new Random();
+	private final Random random = new Random();
 
 	@Autowired
 	private StreamBridge streamBridge;

tools/kafka/docker-compose/README.adoc

@@ -17,6 +17,26 @@ docker-compose -f ./kafka-cluster.yml down
 
 TIP: To start the containers in the foreground simply add `-d` to the start command. To stop containers that are running in the foreground simple issue `CTRL-C` command.
 
+Running a quick test:
+
+Now that we started our clusters, let us try to produce data to a topic on the cluster and then consume from it.
+
+To produce data to a topic named `input-topic`:
+
+```
+docker exec -it broker1 /bin/kafka-console-producer --bootstrap-server localhost:29091 --topic input-topic
+```
+
+Note the use of bootstrap-server at `localhost:29091`.
+This port is internal to the docker container.
+
+To consume from the `input-topic`:
+
+```
+docker exec -it broker1 /bin/kafka-console-consumer --bootstrap-server localhost:29091 --topic input-topic
+```
+
+Enter some data on the producer console and hit enter, you should see the same data gets echoed on the consumer console.
 
 === Control Center UI
 Runs a https://docs.confluent.io/platform/current/control-center/index.html[Confluent Control Center] that exposes a UI at http://locahost:9021.