Add a link to the migration guide

[harrietgrace]

What type of PR is this?
/kind documentation

What does this PR do / why we need it:
Updating Readme to include a link to the existing migration guide.

[openshift-ci]

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[openshift-ci]

Hi @harrietgrace. Thanks for your PR.

I'm waiting for a redhat-developer 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.

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