Add example zombienet network file and instructions (#1839)

* Add simple example on how to spin up network.

* Update readme

* Remove unnecessary prefix

* Improve folder structure

* Add link to file

* Fix paths in readme

* Update README.md

Co-authored-by: Bastian Köcher <git@kchr.de>

Co-authored-by: Bastian Köcher <git@kchr.de>