updated link to mailing list

Author: nchammas <nicholas.chammas@gmail.com>

Closes #923 from nchammas/patch-1 and squashes the following commits:

65c4d18 [nchammas] updated link to mailing list

(cherry picked from commit 23ae36630aa5e6ca4a86e32912a99408129dff7b)
Signed-off-by: Reynold Xin <rxin@apache.org>