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>

zombienet/examples/small_network.toml

@@ -0,0 +1,25 @@
+[relaychain]
+default_image = "parity/polkadot:latest"
+default_command = "polkadot"
+chain = "rococo-local"
+
+  [[relaychain.nodes]]
+  name = "alice"
+  validator = true
+
+  [[relaychain.nodes]]
+  name = "bob"
+  validator = true
+
+[[parachains]]
+id = 2000
+cumulus_based = true
+chain = "statemine-local"
+
+  # run charlie as parachain collator
+  [[parachains.collators]]
+  name = "charlie"
+  validator = true
+  image = "parity/polkadot-parachain:latest"
+  command = "polkadot-parachain"
+  args = ["--force-authoring"]