tekton: conftest task

[garethahealy]

Can this be extended?

• https://github.com/tektoncd/catalog/tree/master/task/conftest/0.1

Can we provide more docs/examples linked back to this repo?

One use case I see not doc'd is being able to pull policies. Need to do a bit of digging to see if this can be done out-of-box but just needs docing.