[Docs] Run tests like in contributing guide

The Contributing to Spark guide [recommends](https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark#ContributingtoSpark-AutomatedTesting) running tests by calling `./dev/run-tests`. The README should, too.

`./sbt/sbt test` does not cover Python tests or style tests.

Author: nchammas <nicholas.chammas@gmail.com>

Closes #2149 from nchammas/patch-2 and squashes the following commits:

2b3b132 [nchammas] [Docs] Run tests like in contributing guide

(cherry picked from commit 73b3089b8d2901dab11bb1ef6f46c29625b677fe)
Signed-off-by: Reynold Xin <rxin@apache.org>