MINOR: add upgrade not for group.initial.rebalance.delay.ms

Add a new entry in upgrade.html for `group.initial.rebalance.delay.ms`

Author: Damian Guy <damian.guy@gmail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>, Guozhang Wang <wangguoz@gmail.com>

Closes #3189 from dguy/relabance-delay

docs/upgrade.html

@@ -70,6 +70,12 @@
     <li>Added user headers support through a new Headers interface providing user headers read and write access.</li>
     <li>ProducerRecord and ConsumerRecord expose the new Headers API via <code>Headers headers()</code> method call.</li>
     <li>ExtendedSerializer and ExtendedDeserializer interfaces are introduced to support serialization and deserialization for headers. Headers will be ignored if the configured serializer and deserializer are not the above classes.</li>
+    <li>A new config,<code>group.initial.rebalance.delay.ms</code>, was introduced.
+        This config specifies the time, in milliseconds, that the <code>GroupCoordinator</code> will delay the initial consumer rebalance.
+        The rebalance will be further delayed by the value of <code>group.initial.rebalance.delay.ms</code> as new members join the group, up to a maximum of <code>max.poll.interval.ms</code>.
+        The default value for this is 3 seconds.
+        During development and testing it might be desirable to set this to 0 inorder to not delay test execution time.
+    </li>
 </ul>
 
 <h5><a id="upgrade_1100_new_protocols" href="#upgrade_1100_new_protocols">New Protocol Versions</a></h5>