selftests: drv-net: update the README

I have added some instructions for driver authors on the NIPA wiki:
  https://github.com/linux-netdev/nipa/wiki/Guidance-for-test-authors
last year. Given the increasingly common use of LLMs let's add those
in tree as well. Hopefully this will decrease the number of review
comments we have to give to AI-assisted noobs.

While at it sync the overall instructions with what's on the GitHub
as well.

Link: https://patch.msgid.link/20260303213626.2320308-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>