Adding note on how to upgrade from Gerrit v3.9.0

Change-Id: Iac967fe1dadced6fe4643077561764926aa3d7b6

pages/site/releases/3.9.md

@@ -119,6 +119,18 @@ and fail to locate changes imported from Gerrit servers with a different server-
 
 ## Important notes
 
+### Upgrade from Gerrit v3.9.0 (defective release)
+
+The release of Gerrit v3.9.0 has been impacted by the bad merge of
+[Change 394445](https://gerrit-review.googlesource.com/c/gerrit/+/394445)
+due to the ["implicit merge on moved changes issue"](https://issues.gerritcodereview.com/issues/40009784)
+that has pulled 64 unwanted commits from the master branch.
+As a consequence, the version has been declared defective and withdrawn.
+
+The upgrade from 3.9.0 to the following 3.9.\* patch-releases needs to
+follow the instructions of the _"Offline upgrade"_ because it contains an
+incompatible Lucene Engine that won't be able to read the legacy index data.
+
 ### Schema and index changes
 
 This release doesn't contain schema changes.