Floodlight doesn't have L3 functionality

Document that, when using BigSwitch/Floodlight plugin with Floodlight,
you need to use quantum-l3-agent since Floodlight doesn't
provide L3 functionality.

Cherry-picked from https://review.openstack.org/32080

Change-Id: Iaec2d58615530c21fdc0c6532a1f6786ff4f3a5a
Merge "Floodlight doesn't have L3 functionality" into stable/grizzly