SPARK-4457. Document how to build for Hadoop versions greater than 2.4

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3322 from sryza/sandy-spark-4457 and squashes the following commits:

5e72b77 [Sandy Ryza] Feedback
0cf05c1 [Sandy Ryza] Caveat
be8084b [Sandy Ryza] SPARK-4457. Document how to build for Hadoop versions greater than 2.4