Per request by Reynold adding small note about proper sequencing of build then test.

Author: Stephen Boesch <javadba@gmail.com>

Closes #1615 from javadba/docs and squashes the following commits:

6c3183e [Stephen Boesch] Moved updated testing blurb per PWendell
5764757 [Stephen Boesch] SPARK-2712 - Add a small note to maven doc that mvn package must happen before test