Add the missing imagePullSecret configuration guidance for a developer

[xiujuan95]

Changes

Especially for this issue. Add the missing imagePullSecret configuration guidance for a developer who wants to install tekton-pipelines use ko and push images to a private registry.

/kind documentation

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices
• Release notes block has been filled in or deleted (only if no user facing changes)

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task to build and release this image.

Reviewer Notes

If API changes are included, additive changes must be approved by at least two OWNERS and backwards incompatible changes must be approved by more than 50% of the OWNERS, and they must first be added in a backwards compatible way.

Release Notes

[tekton-robot]

Hi @xiujuan95. Thanks for your PR.

I'm waiting for a tektoncd 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.

[ghost]

/ok-to-test

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg

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• OWNERS [sbwsg]

Approvers can indicate their approval by writing /approve in a comment
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[pritidesai]

thanks @xiujuan95 for adding this note, this will be very helpful 🙏

I haven't tested imagePullSecrets myself but the changes look great.

/retest

[pritidesai]

Closes #3693

[pritidesai]

closing and reopening to trigger checks 😞

[pritidesai]

/lgtm

[xiujuan95]

@pritidesai Thanks for your attention!