Attempt at making the driver-worker networking requirement more explicit and up-front in the documentation (see https://issues.apache.org/jira/browse/SPARK-6343).

Update cluster overview diagram to show connections from workers to driver. Add a bullet below about how driver listens / accepts connections from workers.

Author: Peter Parente <pparent@us.ibm.com>

Closes #5382 from parente/SPARK-6343 and squashes the following commits:

0b2fb9d [Peter Parente] [SPARK-6343] Doc driver-worker network reqs