Documentation: Add documentation for TSO and GSO features

This document is a starting point for defining the TSO and GSO features.
The whole thing is starting to get a bit messy so I wanted to make sure we
have notes somwhere to start describing what does and doesn't work.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>