net: Mention net-next status web page in netdev-FAQ.txt

According to
  https://www.mail-archive.com/netdev@vger.kernel.org/msg177411.html
there is a status page available at
  http://vger.kernel.org/~davem/net-next.html


to obtain the current status of the net-next tree.  Let's add this
information to the netdev FAQ.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: David S. Miller <davem@davemloft.net>