[SQL][Doc] Keep Spark SQL README.md up to date

marmbrus
Update README.md to be consistent with Spark 1.1

Author: Liquan Pei <liquanpei@gmail.com>

Closes #2706 from Ishiihara/SparkSQL-readme and squashes the following commits:

33b9d4b [Liquan Pei] keep README.md up to date