Added short descriptions of the generic network interface code.

src/netif/FILES

@@ -0,0 +1,28 @@
+This directory contains generic network interface device drivers that
+don't contain any hardware or architecture specific code. The files
+are:
+
+etharp.c
+          Implementes the ARP (Address Resolution Protocol) over
+          Ethernet. The code in this file should be used together with
+          Ethernet device drivers.
+
+ethernetif.c
+          An example of how an Ethernet device driver could look. This
+          file can be used as a "skeleton" for developing new Ethernet
+          network device drivers. It uses the etharp.c ARP code.
+
+loopif.c
+          An example network interface that shows how a "loopback"
+          interface would work. This is not really intended for actual
+          use, but as a very basic example of how initialization and
+          output functions work.
+
+slipif.c
+          A generic implementation of the SLIP (Serial Line IP)
+          protocol. It requires a sio (serial I/O) module to work.
+
+tcpdump.c
+          Not a network interface driver, but some code that parses a
+          TCP/IP packet and outputs printfs in an tcpdump-like
+          way. Only used for debugging.