YARN-2317. Updated the document about how to write YARN applications. Contributed by Li Lu.
svn merge --ignore-ancestry -c 1617594 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1617597 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6830: Merging r1617598 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1617599 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6836: Merging r1617603 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1617604 13f79535-47bb-0310-9956-ffa450edef68