Update README regarding overlapping cidr limitation
Update README regarding overlapping cidr limitation

Overlapping cidrs are supported only via passing pool names using
docker options. Same is being updated in README.

Change-Id: Ia59152e1ddc61560b1ce4af807621add337cf979
Closes-bug: #1596183

celebdor/kuryr-libnetwork,celebdor/kuryr-libnetwork,celebdor/kuryr,celebdor/kuryr,celebdor/kuryr-libnetwork,openstack/kuryr,openstack/kuryr