Add V1 Migration Guide Link in tektoncd/pipeline Readme

[JeromeJu]

Changes

This commit adds the v1 migration guide in readme.md for better visibility of the latest migrating link of pipeline.

/kind documentation

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• [n/a] Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [n/a] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• [n/a] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[afrittoli]

/approve

[jerop]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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