Update documentation for work around for SPARK-1384

This is to workaround accessing secure hdfs from spark-shell in yarn-client mode.  Note this only applies to branch-0.9 and is intended to be included in the documentation for 0.9.1. The real fix after 0.9.1 is included in https://github.com/apache/spark/pull/287

Author: Thomas Graves <tgraves@apache.org>

Closes #314 from tgravescs/docFix09rc3 and squashes the following commits:

222e848 [Thomas Graves] Update documentation for work around for SPARK-1384