selftests: forwarding: README: Mention defer, adf_

Mention how it would be nice if new code used defer. Also if it does that
in dirtying helpers, how it would be nice if these were named adf_*.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/0764bdb9266cd516da23ddeec110e01118cf981e.1758821127.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>