stable_kernel_rules: Add pointer to netdev-FAQ for network patches

Stable_kernel_rules should point submitters of network stable patches to the
netdev_FAQ.txt as requests for stable network patches should go to netdev
first.

Signed-off-by: Dave Chiluk <chiluk@canonical.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>