Add tkn to catalog.

Adds a Task to the catalog for the [Tekton CLI](https://github.com/tektoncd/cli).

tkn/README.md

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+# tkn
+
+This task performs operations on Tekton resources using
+[`tkn`](https://github.com/tektoncd/cli).
+
+## Install the Task
+
+```
+kubectl apply -f https://raw.githubusercontent.com/tektoncd/catalog/master/tkn/tkn.yaml
+```
+
+## Inputs
+
+### Parameters
+
+name      | description                                 | default
+--------- | ------------------------------------------- | -------
+tkn-image | `tkn` CLI container image to run this task. | gcr.io/tekton-releases/dogfooding/tkn
+ARGS      | The arguments to pass to the `tkn` CLI.     | `["help"]`

tkn/tkn.yaml

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+---
+apiVersion: tekton.dev/v1alpha1
+kind: Task
+metadata:
+  name: tkn
+spec:
+  inputs:
+    params:
+      - name: tkn-image
+        description: tkn CLI container image to run this task
+        default: gcr.io/tekton-releases/dogfooding/tkn
+      - name: ARGS
+        type: array
+        description: tkn CLI arguments to run
+        default: ["help"]
+  steps:
+    - name: tkn
+      image: "$(inputs.params.tkn-image)"
+      command: ["/usr/local/bin/tkn"]
+      args: ["$(inputs.params.ARGS)"]