[Versioning] Add CHANGELOG entries for version 3.9.1.

Signed-off-by: Giacomo Marciani <[email protected]>

Author: gmarciani

CHANGELOG.md

@@ -3,6 +3,13 @@ aws-parallelcluster-cookbook CHANGELOG
 
 This file is used to list changes made in each version of the AWS ParallelCluster cookbook.
 
+3.9.1
+------
+
+**BUG FIXES**
+- Fix an issue that may cause data loss when a shared storage is unmounted as part of a cluster update
+  while it is in use and with no backup strategy in place.
+
 3.9.0
 ------