add demo documentation

DEMO.md

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+# Start Kafka service
+The following commands will start a container with Kafka and Zookeeper running on mapped ports 2181 (Zookeeper) and 9092 (Kafka).
+```
+docker pull spotify/kafka
+docker run -d -p 2181:2181 -p 9092:9092 --env ADVERTISED_HOST=kafka --env ADVERTISED_PORT=9092 --name kafka spotify/kafka
+```
+
+[Why Spotify?](https://github.com/spotify/docker-kafka#why)
+
+`ADVERTISTED_HOST` was set to `kafka`, which will allow other containers to be able to run Producers and Consumers.
+
+Setting `ADVERTISED_HOST` to `localhost`, `127.0.0.1`, or `0.0.0.0` will work great only if Producers and Consumers are started within the `kafka` container itself, or if you are using DockerForMac (like me) and you want to run Producers and Consumers from OSX.
+These are far less interesting use cases though, so we'll start Producers and Consumers from other containers.
+
+We need to use an IP address or hostname in order for the `kafka` service to be reachable from another container. IP address is not known before the container is started, so we have to choose a hostname, and I chose `kafka` in this example.
+
+# Create a topic
+```
+docker exec kafka /opt/kafka_2.11-0.10.1.0/bin/kafka-topics.sh --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic test
+```
+
+output:
+```
+Created topic "test".
+```
+
+# List topics
+```
+docker exec kafka /opt/kafka_2.11-0.10.1.0/bin/kafka-topics.sh --list --zookeeper localhost:2181
+```
+
+output:
+```
+test
+```
+
+# Start a producer (in a new terminal window)
+This command will run an unnamed instance of `spotify/kafka` linked to the `kafka` service, start a producer, and wait for newline-delimited input until you quit (which destroys the container):
+```
+docker run -it --rm --link kafka spotify/kafka /opt/kafka_2.11-0.10.1.0/bin/kafka-console-producer.sh --broker-list kafka:9092 --topic test
+```
+
+# Start a consumer (in a new terminal window)
+This command will start an unnamed instance of `spotify/kafka` linked to the `kafka` service, start a consumer, display existing messages from the `test` topic, and wait for new messages until you quit (which destroys the container):
+```
+docker run -it --rm --link kafka spotify/kafka /opt/kafka_2.11-0.10.1.0/bin/kafka-console-consumer.sh --bootstrap-server kafka:9092 --topic test --from-beginning
+```
+
+# Send messages
+Send some newline-delimited messages in the Producer terminal window. The messages appear in the Consumer terminal window.