HDFS-7610. Add CHANGES.txt

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

@@ -747,6 +747,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-7496. Fix FsVolume removal race conditions on the DataNode by
     reference-counting the volume instances (lei via cmccabe)
 
+    HDFS-7610. Fix removal of dynamically added DN volumes (Lei (Eddy) Xu via
+    Colin P. McCabe)
+
 Release 2.6.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES