docs: netdev: add a cheat sheet for the rules

Summarize the rules we see broken most often and which may
be less familiar to kernel devs who are used to working outside
of netdev.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>