d: add README.Debian

This commit includes a bit of general information about nftables.

This was originally posted by Patric McHardy <kaber@trash.net> at the Netfilter
devel mailing list <netfilter-devel@vger.kernel.org>.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>