Documentation: add networking/netdev-FAQ.txt

Documentation: add networking/netdev-FAQ.txt

A collection of expectations and operational details about how
networking development takes place in the context of the netdev
mailing list.

The content is meant to capture specific items that are unique
to netdev workflow, and not re-document generic linux expectations
that are already captured elsewhere.

This was originally proposed[1] as a regular posting mailing list
FAQ, but it probably is more universally accessible here in tree.

[1] https://lwn.net/Articles/559211/

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>