Add mesos specific configurations into doc

Author: Timothy Chen <tnachen@gmail.com>

Closes #3349 from tnachen/mesos_doc and squashes the following commits:

737ef49 [Timothy Chen] Add TOC
5ca546a [Timothy Chen] Update description around cores requested.
26283a5 [Timothy Chen] Add mesos specific configurations into doc

(cherry picked from commit d9956f86ad7a937c5f2cfe39eacdcbdad9356c30)
Signed-off-by: Reynold Xin <rxin@databricks.com>