docs: netdev: add a note on selftest posting

We haven't had much discussion on the list about this, but
a handful of people have been confused about rules on
posting selftests for fixes, lately. I tend to post fixes
with their respective selftests in the same series.
There are tradeoffs around size of the net tree and conflicts
but so far it hasn't been a major issue.

Signed-off-by: Jakub Kicinski <[e-mail]>
Acked-by: Cong Wang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

Documentation/process/maintainer-netdev.rst

···
 Posting as one thread is discouraged because it confuses patchwork
 (as of patchwork 2.2.2).
 
+Co-posting selftests
+--------------------
+
+Selftests should be part of the same series as the code changes.
+Specifically for fixes both code change and related test should go into
+the same tree (the tests may lack a Fixes tag, which is expected).
+Mixing code changes and test changes in a single commit is discouraged.
+
 Preparing changes
 -----------------