Create Pipeline for releases

[bobcatfish]

Expected Behavior

We should be testing and releasing Pipelines with Pipelines.

We should have a Pipeline that performs these operations, in order:

1. Builds + tags images (Create Task for building and tagging Tekton Pipelines release images #528)
2. Publishes a release.yaml with those images (Create Task for publishing Tekton Pipeline release yaml #529)
3. Creates a GitHub release (Create Task for creating GitHub release #530)

Actual Behavior

We currently use prow and knative release scripts for releasing.

Additional Info

• Actually triggering this is not in scope for this issue, it's fine to test manually. For now we can assume we'll run this manually when we want to release; eventually we will create a similar Pipeline for nightly releases Now that Prow has support for Tekton Pipelines (Now that Prow has support for Tekton Pipelines (Add a controller which creates a tekton pipeline run from a prow job kubernetes/test-infra#11888), ), it should be possible to invoke the Pipelines with Prow! (note that the configuration for this lives in the plumbing repo)
• Eventually we'll want this Pipeline to trigger tests as well but since we don't have a Pipeline for this yet, we can for now create a Release Pipeline that only knows how to Release
• The PR should include docs on how to use this

[bobcatfish]

With the Pipeline added by @vdemeester in #1095 i think we can call this done! We can always add in the GitHub Task once we have it.