Merge -c 1493570 from trunk to branch-2.1-beta to fix MAPREDUCE-5184. Document compatibility for MapReduce applications in hadoop-2 vis-a-vis hadoop-1. Contributed by Zhijie Shen.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1493572 13f79535-47bb-0310-9956-ffa450edef68