selftests: drv-net: extend the README with more info and example

Add more info to the README. It's also now copied to GitHub for
increased visibility:

 https://github.com/linux-netdev/nipa/wiki/Running-driver-tests



Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240425222341.309778-2-kuba@kernel.org


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