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/trunk@1493570 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-9235-trunk-gd