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>

cumulus/zombienet/tests/0004-runtime_upgrade.toml

@@ -0,0 +1,33 @@
+[relaychain]
+default_image = "{{RELAY_IMAGE}}"
+default_command = "polkadot"
+default_args = [ "-lparachain=debug" ]
+
+chain = "rococo-local"
+
+  [[relaychain.nodes]]
+  name = "alice"
+  validator = true
+
+  [[relaychain.nodes]]
+  name = "bob"
+  validator = true
+
+[[parachains]]
+id = 2000
+cumulus_based = true
+
+  # run charlie as parachain collator
+  [[parachains.collators]]
+  name = "charlie"
+  validator = true
+  image = "{{COL_IMAGE}}"
+  command = "test-parachain"
+  args = ["-lparachain=debug"]
+
+  # Run dave as parachain full node
+  [[parachains.collators]]
+  name = "dave"
+  validator = false
+  image = "{{COL_IMAGE}}"
+  command = "test-parachain"