doc: add varReference example

[AlphaWong]

Missing example.

I think it is better to have an example of varReference in var section. As I cannot find any resource from official doc and people are keep asking it in issue and it is a good feature of Kustomize.

related

1. How to update varreference #2704
2. https://github.com/kubernetes-sigs/kustomize/blob/master/examples/transformerconfigs/crd/README.md
3. Vars substitution does not work in ConfigMap #1250 (comment)

[k8s-ci-robot]

Welcome @AlphaWong!

It looks like this is your first PR to kubernetes-sigs/kustomize 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes-sigs/kustomize has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[k8s-ci-robot]

Hi @AlphaWong. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: AlphaWong
To complete the pull request process, please assign droot after the PR has been reviewed.
You can assign the PR to them by writing /assign @droot in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Shell32-Natsu]

/ok-to-test
/lgtm

Thanks a lot for this!