Removed reference to incubation in README.md.

Author: Reynold Xin <rxin@apache.org>

Closes #1 from rxin/readme and squashes the following commits:

b3a77cd [Reynold Xin] Removed reference to incubation in README.md.
(cherry picked from commit 84f7ca138165ca413897dada35c602676b0a614f)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>