documented the remaining ones (sysdep/cf/README).

Available configurations:

   o  linux-20: Old Linux interface via /proc/net/route (selected by default
		on pre-2.1 kernels).
   o  linux-21: Old Linux interface, but device routes handled by the
		kernel (selected by default for 2.1 and newer kernels).
   o  linux-22: Linux with Netlink (I play with it a lot yet, so it isn't
		a default).
   o  linux-ipv6: Prototype config for IPv6 on Linux. Not functional yet.