update CHANGES.txt to add YARN-3697 to 2.6.4 release

(cherry picked from commit 96d8f1d6f30299831dde3985a6a3bd08cb36fd38)

hadoop-yarn-project/CHANGES.txt

@@ -2078,6 +2078,9 @@ Release 2.6.4 - UNRELEASED
   YARN-4452. NPE when submit Unmanaged application. (Naganarasimha G R
   via junping_du)
 
+  YARN-3697. FairScheduler: ContinuousSchedulingThread can fail to shutdown.
+  (Zhihai Xu via kasha)
+
 Release 2.6.3 - 2015-12-17
 
   INCOMPATIBLE CHANGES