README update

Author: Reynold Xin <rxin@apache.org>

Closes #443 from rxin/readme and squashes the following commits:

16853de [Reynold Xin] Updated SBT and Scala instructions.
3ac3ceb [Reynold Xin] README update

(cherry picked from commit 28238c81d9d81dba5d880cbd7ee910ec326bdc79)
Signed-off-by: Reynold Xin <rxin@apache.org>