igb.txt: Add igb documentation

Add documentation for the igb networking driver.

  v2:
   - Removed trailing white space
   - Removed Ethtool version info
   - Removed LRO kernel 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>