BigSwitch/Floodlight Quantum Plugin README,VIF

Adding a reference to the BigSwitch/Floodlight REST Proxy Quantum Plugin
README. Also adding a section for the VIF configuration.

Bug #1116570

Change-Id: I9fe229518dd59628687eb6ddb952da5509b8bbe4
Merge "BigSwitch/Floodlight Quantum Plugin README,VIF"