doc: add library documentation from Brad Fisher

doc: add library documentation from Brad Fisher

This patch adds the documentation that Brad Fisher <brad@info-link.net>
made time ago, you can get the original post from:

http://lists.netfilter.org/pipermail/netfilter-devel/2006-February/023286.html

This patch contains documentation that I did not have time to review in deep
yet. However, several developers has refered to this documentation as a good
kick-off to start developing applications for libnetfilter_queue.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>