Update CHANGES.txt to reflect commit of MR-6273 to branch-2.6

(cherry picked from commit 6ae9efaf5949bd5a5f4fd99b5777ce8f6d7f3a2c)

hadoop-mapreduce-project/CHANGES.txt

@@ -278,6 +278,10 @@ Release 2.6.3 - UNRELEASED
 
   BUG FIXES
 
+    MAPREDUCE-6273. HistoryFileManager should check whether summaryFile exists to 
+    avoid FileNotFoundException causing HistoryFileInfo into MOVE_FAILED state.
+    (zhihai xu via devaraj)
+
 Release 2.6.2 - 2015-10-21
 
   INCOMPATIBLE CHANGES