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

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

README.md

@@ -69,7 +69,7 @@ Many of the example programs print usage help if no params are given.
 Testing first requires [building Spark](#building-spark). Once Spark is built, tests
 can be run using:
 
-    ./sbt/sbt test
+    ./dev/run-tests
 
 ## A Note About Hadoop Versions