Update CHANGES.txt to add YARN-3857 and YARN-3535 to branch-2.6

hadoop-yarn-project/CHANGES.txt

@@ -2036,6 +2036,12 @@ Release 2.6.4 - UNRELEASED
 
   BUG FIXES
 
+  YARN-3857: Memory leak in ResourceManager with SIMPLE mode.
+  (mujunchao via zxu)
+
+  YARN-3535. Scheduler must re-request container resources when RMContainer transitions
+  from ALLOCATED to KILLED (rohithsharma and peng.zhang via asuresh)
+
 Release 2.6.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES