igbvf.txt: Add igbvf Documentation

Adds documentation for the igbvf (igb virtual function driver).

  v2:
   - Removed trailing white space
   - Removed Ethtool version info

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>