HDFS-7496: add to CHANGES.txt

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -744,6 +744,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-7641. Update archival storage user doc for list/set/get block storage
     policies. (yliu)
 
+    HDFS-7496. Fix FsVolume removal race conditions on the DataNode by
+    reference-counting the volume instances (lei via cmccabe)
+
 Release 2.6.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES