SPARK-1873: Add README.md file when making distributions

Author: Patrick Wendell <pwendell@gmail.com>

Closes #818 from pwendell/reamde and squashes the following commits:

4020b11 [Patrick Wendell] SPARK-1873: Add README.md file when making distributions

(cherry picked from commit 4ce47932)
Signed-off-by: Matei Zaharia <matei@databricks.com>