selftests: forwarding: README: Document customization

That any sort of customization is possible at all, let alone how it should
be done, is currently not at all clear. Document the whats and hows in
README.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/e819623af6aaeea49e9dc36cecd95694fad73bb8.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>