updating CHANGES.txt after committing MAPREDUCE-5333,HADOOP-9661,HADOOP-9355,HADOOP-9673,HADOOP-9414,HADOOP-9416,HDFS-4797,YARN-866,YARN-736,YARN-883 to 2.1-beta branch

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502074 13f79535-47bb-0310-9956-ffa450edef68
YARN-569. Add support for requesting and enforcing preemption requests via
a capacity monitor. Contributed by Carlo Curino, Chris Douglas


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502084 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd