Use --context in cheat sheet commands 🧑‍🔬

This change is trying to balance a few things:
* If you've never done a release before, the cheat sheet is useful
  but doesn't have all the info you might need. This change is trying
  to give new folks enough to be able to make changes against the right
  cluster but also not bog down the "cheat sheet" nature of the list
* If you connect to the dogfooding cluster, by default all kubectl (and
  tkn) commands will be run against it, which can be dangerous if you
  forget

So this change updates the examples to assume you have a --context setup
that you can use to easily refer to the dogfooding cluster and includes
some very brief instructions at the bottom of the doc on how to set this
up.

tekton/release-cheat-sheet.md

@@ -4,16 +4,9 @@ These steps provide a no-frills guide to performing an official release
 of Tekton Pipelines. To follow these steps you'll need a checkout of
 the pipelines repo, a terminal window and a text editor.
 
-1. `cd` to root of Pipelines git checkout.
-
-1. Configure `kubectl` to use the  dogfooding cluster
-   [as the default context](https://kubernetes.io/docs/tasks/access-application-cluster/configure-access-multiple-clusters/)
-   (**in the final step we will configure the default context to point to something else; until then be aware
-    that any kubctl commands you run will act on the dogfooding cluster**)
+1. [Setup a context to connect to the dogfooding cluster](#setup-dogfooding-context) if you haven't already.
 
-    ```bash
-    gcloud container clusters get-credentials dogfooding --zone us-central1-a --project tekton-releases
-    ```
+1. `cd` to root of Pipelines git checkout.
 
 1. Select the commit you would like to build the release from, most likely the
    most recent commit at https://github.com/tektoncd/pipeline/commits/master
@@ -35,7 +28,7 @@ the pipelines repo, a terminal window and a text editor.
         value: # The commmit you selected in the last step, e.g. 33e0847e67fc9804689e50371746c3cdad4b0a9d
     ```
 
-1. Apply file you just made to the dogfooding cluster: `kubectl apply -f your-pipeline-resource-file.yaml`
+1. Apply file you just made to the dogfooding cluster: `kubectl --context dogfooding apply -f your-pipeline-resource-file.yaml`
 
 1. Create environment variables for bash scripts in later steps.
 
@@ -48,15 +41,15 @@ the pipelines repo, a terminal window and a text editor.
 1. Confirm commit SHA matches what you want to release.
 
     ```bash
-    kubectl get pipelineresource "$TEKTON_RELEASE_GIT_RESOURCE" -o=jsonpath="{'Target Revision: '}{.spec.params[?(@.name == 'revision')].value}{'\n'}"
+    kubectl --context dogfooding get pipelineresource "$TEKTON_RELEASE_GIT_RESOURCE" -o=jsonpath="{'Target Revision: '}{.spec.params[?(@.name == 'revision')].value}{'\n'}"
     ```
 
 1. Execute the release pipeline.
 
     **If you are backporting include this flag: `--param=releaseAsLatest="false"`**
 
     ```bash
-    tkn pipeline start \
+    tkn --context dogfooding pipeline start \
       --param=versionTag="${TEKTON_VERSION}" \
       --param=imageRegistry="${TEKTON_IMAGE_REGISTRY}" \
       --serviceaccount=release-right-meow \
@@ -95,7 +88,7 @@ the pipelines repo, a terminal window and a text editor.
     1. Execute the Draft Release task.
 
     ```bash
-    tkn task start \
+    tkn --context dogfooding task start \
       -i source="${TEKTON_RELEASE_GIT_RESOURCE}" \
       -i release-bucket=pipeline-tekton-bucket \
       -p package="${TEKTON_PACKAGE}" \
@@ -107,8 +100,7 @@ the pipelines repo, a terminal window and a text editor.
 
     1. Watch logs of create-draft-release
 
-    1. On successful completion, a URL will be logged. Visit that URL and sort the
-    release notes.
+    1. On successful completion, a URL will be logged. Visit that URL and look through the release notes.
       1. Manually add upgrade and deprecation notices based on the generated release notes
       1. Double-check that the list of commits here matches your expectations
          for the release. You might need to remove incorrect commits or copy/paste commits
@@ -122,22 +114,16 @@ the pipelines repo, a terminal window and a text editor.
 
 1. Push & make PR for updated `README.md`
 
-1. **Important: Stop pointing `kubectl` at dogfooding cluster.**
-
-    ```bash
-    kubectl config use-context my-dev-cluster
-    ```
-
 1. Test release that you just made.
 
     ```bash
     # Test latest
-    kubectl apply --filename https://storage.googleapis.com/tekton-releases/pipeline/latest/release.yaml
+    kubectl --context dogfooding apply --filename https://storage.googleapis.com/tekton-releases/pipeline/latest/release.yaml
     ```
 
     ```bash
     # Test backport
-    kubectl apply --filename https://storage.googleapis.com/tekton-releases/pipeline/previous/v0.11.2/release.yaml
+    kubectl --context dogfooding apply --filename https://storage.googleapis.com/tekton-releases/pipeline/previous/v0.11.2/release.yaml
     ```
 
 1. Announce the release in Slack channels #general and #pipelines.
@@ -146,3 +132,25 @@ the pipelines repo, a terminal window and a text editor.
 to use the new release by updating the `RELEASE_YAML` link in [e2e-tests.sh](https://github.com/tektoncd/catalog/blob/master/test/e2e-tests.sh).
 
 Congratulations, you're done!
+
+## Setup dogfooding context
+
+1. Configure `kubectl` to connect to
+   [the dogfooding cluster](https://github.com/tektoncd/plumbing/blob/master/docs/dogfooding.md):
+
+    ```bash
+    gcloud container clusters get-credentials dogfooding --zone us-central1-a --project tekton-releases
+    ```
+
+1. Give [the context](https://kubernetes.io/docs/tasks/access-application-cluster/configure-access-multiple-clusters/)
+   a short memorable name such as `dogfooding`:
+
+   ```bash
+   kubectl config rename-context gke_tekton-releases_us-central1-a_dogfooding dogfoodin
+   ```
+
+1. **Important: Switch `kubectl` back to your own cluster by default.**
+
+    ```bash
+    kubectl config use-context my-dev-cluster
+    ```