eli is testing a demo of the kustomize replacement feature. You can find kustomize docs on replacement here.
Normally in Kustomize we include the "base" and the "overlay" resources. Here we don't care about the "overlay" resources except in as much as they provide us a way to swap in some data to the "base". However, Kustomize eschews including resources for a yml you don't want built, but we need to include the resource yml for the replacement command to work. That's why "file" and "overlay" are different types.
To run, just kustomize build . The full output as of this writing is:
$ kustomize build .
apiVersion: v1
kind: Namespace
metadata:
name: file
stuff:
- name: one
runbook: url
- name: two
runbook: IT WORKS!
- name: three
runbook: url
---
apiVersion: v1
data:
- name: two
runbook: IT WORKS!
kind: Configmap
metadata:
name: runbooks