Add flakes as experimental feature to Nix config (#607)

* Add flakes as experimental feature to Nix config

* Document parallel builds

* typo

Author: sbernauer

modules/contributor/pages/testing-on-kubernetes.adoc

@@ -34,9 +34,17 @@ If you don't want to run an arbitrary shellscript directly from the web, have a
 
 After this is done you also need to add a setting to your Nix config in `/etc/nix/nix.conf`:
 ----
-experimental-features = nix-command
+experimental-features = nix-command flakes
 ----
 
+It is also recommended to enable parallel builds by also adding the following configs:
+----
+max-jobs = 24
+cores = 8
+----
+
+Please adjust the number of `cores` to your system and set the `max-jobs` to e.g. the third of that.
+
 Just installing Nix does not affect your system much, as it keeps all its configuration and installed packages separate from other package managers and you won't even notice it is there, unless you actually start using it.
 
 == Using