Add nop-image to release instructions

[bobcatfish]

Changes

In #3025 the nop-image was added to our release pipeline, this commit
updates our README to include an example that sets it.

Also removed the reference to resources.yaml from the instructions b/c
imo opening that file and modifying it is more confusing than helpful;
feels like it makes more sense to create a new PipelineResource instance
without bothering with that file.

Submitter Checklist

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

• [n/a] 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

NONE

[vdemeester]

/lgtm

[ghost]

Suggest updating the release cheat sheet as well: https://github.com/tektoncd/pipeline/blob/master/tekton/release-cheat-sheet.md

I can do that in a separate PR.

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg, vdemeester

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[dibyom]

/retest

[bobcatfish]

Thanks @sbwsg ! ps can we delete the other README and replace it with the cheatsheet? i didnt know there was one!

[ghost]

can we delete the other README and replace it with the cheatsheet?

I was hesitant to do that originally because of the extra information contained in the README. But I'd definitely be open to it. I'll add something to the productivity WG agenda for next week.

[vdemeester]

/test pull-tekton-pipeline-integration-tests