describe the go get step before cue get go in the tutorial

see cue-lang#95.  Still needs to be done for the docs in cuelang.org website.

Closes cue-lang#180
cuelang/cue#180

GitOrigin-RevId: a3cb34161e7aa24bafb8a5297c99a0c575280936
Change-Id: Ib57b868660239807a6944a133e79f2e92eaab53a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4080
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

doc/tutorial/kubernetes/README.md

@@ -1050,6 +1050,14 @@ of course.
 
 ### Extract CUE templates directly from Kubernetes Go source
 
+In order for `cue get go` to generate the CUE templates from Go sources, you first need to have the sources locally:
+
+```
+$ go get k8s.io/api/apps/v1
+$ go get k8s.io/api/extensions/v1beta1
+$ go get k8s.io/api/apps/v1beta1
+```
+
 ```
 $ cue get go k8s.io/api/core/v1
 $ cue get go k8s.io/api/extensions/v1beta1