MINOR: Update Streams quickstart to create output topic with compaction enabled

Author: Matthias J. Sax <matthias@confluent.io>

Reviewers: Michael G. Noll <michael@confluent.io>, Guozhang Wang <wangguoz@gmail.com>

Closes #3949 from mjsax/minor-update-streams-quickstart

docs/streams/quickstart.html

@@ -129,12 +129,18 @@ Next, we create the input topic named <b>streams-plaintext-input</b> and the out
     --partitions 1 \
     --topic streams-plaintext-input
 Created topic "streams-plaintext-input".
+</pre>
 
+Note: we create the output topic with compaction enabled because the output stream is a changelog stream
+(cf. <a href="#anchor-changelog-output">explanation of application output</a> below).
+
+<pre class="brush: bash;">
 &gt; bin/kafka-topics.sh --create \
     --zookeeper localhost:2181 \
     --replication-factor 1 \
     --partitions 1 \
-    --topic streams-wordcount-output
+    --topic streams-wordcount-output \
+    --config cleanup.policy=compact
 Created topic "streams-wordcount-output".
 </pre>
 
@@ -263,6 +269,7 @@ hello kafka streams
 join kafka summit
 </pre>
 
+<a name="anchor-changelog-output"></a>
 The <b>streams-wordcount-output</b> topic will subsequently show the corresponding updated word counts (see last three lines):
 
 <pre class="brush: bash;">

docs/streams/tutorial.html

@@ -496,6 +496,7 @@
 
     <p>
         We can also write the <code>counts</code> KTable's changelog stream back into another Kafka topic, say <code>streams-wordcount-output</code>.
+        Because the result is a changelog stream, the output topic <code>streams-wordcount-output</code> should be configured with log compaction enabled.
         Note that this time the value type is no longer <code>String</code> but <code>Long</code>, so the default serialization classes are not viable for writing it to Kafka anymore.
         We need to provide overridden serialization methods for <code>Long</code> types, otherwise a runtime exception will be thrown:
     </p>