docs: netdev: offer performance feedback to contributors

Some of us gotten used to producing large quantities of peer feedback
at work, every 3 or 6 months. Extending the same courtesy to community
members seems like a logical step. It may be hard for some folks to
get validation of how important their work is internally, especially
at smaller companies which don't employ many kernel experts.

The concept of "peer feedback" may be a hyperscaler / silicon valley
thing so YMMV. Hopefully we can build more context as we go.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>