MINOR: Add a note about Zstandard compression in the upgrade docs

Author: Jason Gustafson <jason@confluent.io>

Reviewers: Dong Lin <lindong28@gmail.com>

Closes #5817 from hachikuji/add-zstandard-upgrade-notes

docs/upgrade.html

@@ -50,6 +50,10 @@
         and insight by viewing the metrics captured from the <code>AdminClient</code>. For more information
         see <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-324%3A+Add+method+to+get+metrics%28%29+in+AdminClient">KIP-324</a>
     </li>
+    <li>Kafka now supports Zstandard compression from <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-110%3A+Add+Codec+for+ZStandard+Compression">KIP-110</a>.
+        You must upgrade the broker as well as clients to make use of it. Consumers prior to 2.1.0 will not be able to read from topics which use
+        Zstandard compression, so you should not enable it for a topic until all downstream consumers are upgraded. See the KIP for more detail.
+    </li>
 </ul>
 
 <h4><a id="upgrade_2_0_0" href="#upgrade_2_0_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, or 1.1.x to 2.0.0</a></h4>