HADOOP-9279. Document the need to build hadoop-maven-plugins for eclipse and separate project builds. Contributed by Tsuyoshi Ozawa.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1449543 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-9281-trunk-gd