Using CronJobs to automatically clean up completed Runs

[psschwei]

Changes

This PR fixes #479 by providing documentation on how to use a CronJob to clean up old TaskRuns / PipelineRuns. It is based on what @afrittoli implemented in tektoncd/plumbing#442.

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[afrittoli]

Thanks for this!
This looks good - the only missing bit is the cluster resource.
I think we can merge this as it is and then update it in a separate PR.

In fact it would be great to change this so that it does not use a pipeline resource anymore - since now they're deprecated. An alternative could be https://github.com/tektoncd/catalog/tree/master/task/kubeconfig-creator/0.1

/approve
/lgtm

[psschwei]

This looks good - the only missing bit is the cluster resource.
I think we can merge this as it is and then update it in a separate PR.

In fact it would be great to change this so that it does not use a pipeline resource anymore - since now they're deprecated.

Thanks @afrittoli -- will look to get those updates in later today / tomorrow

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