doc/tutorial/kubernetes: add required go mod init step

A Go module is, as of a616925, required in order for cue get go to
resolve package arguments. This step is currently missing from the
Kubernetes tutorial.

There are more issues that need to be fixed as part of the Kubernetes
tutorial, but these are covered in #824.

Change-Id: I703f8159c79cd28cf2e27117b23c200518c2aee5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8962
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>

doc/tutorial/kubernetes/README.md

@@ -77,6 +77,13 @@ We do that later by giving all the same package name.
 $ cue mod init
 ```
 
+We initialize a Go module so that later we can resolve the
+`k8s.io/api/apps/v1` Go package dependency:
+
+```
+$ go mod init example.com
+```
+
 Creating a module also allows our packages import external packages.
 
 Let's try to use the `cue import` command to convert the given YAML files