This provides preliminary documentation pointing out how to use the
Hadoop free builds. I am hoping over time this list can grow to
include most of the popular Hadoop distributions.

Getting more people using these builds will help us long term reduce
the number of binaries we build.

Author: Patrick Wendell <patrick@databricks.com>

Closes #6729 from pwendell/hadoop-provided and squashes the following commits:

1113b76 [Patrick Wendell] [SPARK-6511] [Documentation] Explain how to use Hadoop provided builds