[FAB-4864] Doc update for max file descriptors
[FAB-4864] Doc update for max file descriptors

This change set adds a warning to the README to notify developers that
they may need to increase max file descriptors to run the unit tests.
With all of the new tests, the number of required file descriptors has
increased, and OSX's default is only 256, thus the recent failure.

Change-Id: I0a8c0d9483e2a8dca5122f00640f0665e5a51f40
Signed-off-by: Keith Smith <0f8efcf9aed721c212c631c567e094685b5cc962@us.ibm.com>

saadk10/fabric-ca,saadk10/fabric-ca