Improved igb startup.sh and documentation

* The startup script in `kmod/igb` is enhanced by a parameter to provide an optional interface name. I think it is a better solution than changing it directly in source code depending on the used platform.

* The second part is some simple documentation on how to setup the talker/listener example. I might be helpful for newcomers. Of course, more detailed explanations can and should be added to make it even more helpful.

What do you think?