MINOR: Add missing upgrade doc for 2.8
Reviewers: Manikumar Reddy
</li>
</ul>
+<h4><a id="upgrade_2_8_1" href="#upgrade_2_8_1">Upgrading to 2.8.1 from any version 0.8.x through 2.7.x</a></h4>
+
+<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., <code>2.7</code>, <code>2.6</code>, etc.)</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 version 0.11.0.x or above, 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 (e.g., <code>2.7</code>, <code>2.6</code>, etc.)</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 <code>2.8</code>.
+ </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 2.8 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_280_notable" href="#upgrade_280_notable">Notable changes in 2.8.0</a></h5>
<ul>
<li>
