Update README with dogfooding cluster in place of prow
Update README with dogfooding cluster in place of prow

We now have a dogfooding cluster thanks to the work that afrittoli has
done with our plumbing. The dogfooding cluster can be used for public
releases of Tekton.

This commit replaces mentions of the `prow` cluster with the
`dogfooding` cluster for future release shepherds.

tektoncd/pipeline,tektoncd/pipeline