diff --git a/ReleaseNotes/ReleaseNotes-2.11.txt b/ReleaseNotes/ReleaseNotes-2.11.txt
index 49b1317..f085949 100644
--- a/ReleaseNotes/ReleaseNotes-2.11.txt
+++ b/ReleaseNotes/ReleaseNotes-2.11.txt
@@ -20,9 +20,39 @@
 *WARNING:* This release contains schema changes.  To upgrade:
 ----
   java -jar gerrit.war init -d site_path
+----
+
+Gerrit 2.11 requires a secondary index, which can be created offline
+by running the `reindex` program:
+
+----
   java -jar gerrit.war reindex -d site_path
 ----
 
+If the site that is upgraded already has a secondary index, the
+secondary index can be upgraded online. This is important for large
+sites since running the `reindex` program can take a long time and
+contributes significantly to the downtime that is required for the
+upgrade.
+
+Gerrit 2.11 supports online reindexing only from the index version `10`
+which is the index version of Gerrit 2.10. This means if you come from
+an older release it makes sense to first upgrade to 2.10 and then do
+the upgrade to 2.11 so that you can profit from online reindexing.
+
+In case you are upgrading from 2.10 it is *important* to check *before*
+the upgrade to 2.11 that the index version of your Gerrit 2.10 site is
+`10`. You can check the index version in
+`$site_path/index/gerrit_index.config`. Your Gerrit 2.10 site may run
+with an older index version (e.g. if online reindexing to index version
+`10` is still running or if online reindexing to version `10` has
+failed). In this case you first need to successfully migrate your index
+version of your Gerrit 2.10 site to `10` and only then start with the
+2.11 upgrade. If you start the 2.11 upgrade when the schema version of
+your Gerrit 2.10 site is older than `10`, online reindexing is not
+possible anymore and you need to reindex offline by using the `reindex`
+program.
+
 *WARNING:* Upgrading to 2.11.x requires the server be first upgraded to 2.8 (or
 2.9) and then to 2.11.x. If you are upgrading from 2.8.x or later, you may ignore
 this warning and upgrade directly to 2.11.x.