MINOR: add upgrade text (#7013)

Reviewers: David Arthur <mumrah@gmail.com>

docs/upgrade.html

@@ -28,7 +28,45 @@
 
 <h4><a id="upgrade_2_3_0" href="#upgrade_2_3_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, 1.1.x, 2.0.x or 2.1.x or 2.2.x to 2.3.0</a></h4>
 
-<!-- TODO core-team -->
+<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
+    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>
+
+<p><b>For a rolling upgrade:</b></p>
+
+<ol>
+    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
+        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
+        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
+        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
+        <ul>
+            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
+            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
+                following the upgrade</a> for the details on what this configuration does.)</li>
+        </ul>
+        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, 2.0.x, or 2.1.x, and you have not overridden the message format, then you only need to override
+        the inter-broker protocol version.
+        <ul>
+            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0, 2.1, 2.2).</li>
+        </ul>
+    </li>
+    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
+        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
+        It is still possible to downgrade at this point if there are any problems.
+    </li>
+    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
+        <code>inter.broker.protocol.version</code> and setting it to 2.3.
+    </li>
+    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
+        protocol version, it will no longer be possible to downgrade the cluster to an older version.
+    </li>
+    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
+        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
+        change log.message.format.version to 2.3 on each broker and restart them one by one. Note that the older Scala clients,
+        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
+        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
+        the newer Java clients must be used.
+    </li>
+</ol>
 
 <h5><a id="upgrade_230_notable" href="#upgrade_230_notable">Notable changes in 2.3.0</a></h5>
 <ul>