Add configuration and docs on how to update Prow 

Today I updated Prow as a step toward tektoncd/pipeline#922
- we need an updated version of Prow so that we can have support for
triggering Pipelines! :D

It turned out to be a bit of an ordeal to update Prow, mostly because:
- There were things we'd done a bit differently than the example Prow
  configs
- It turns out if you update Plank without updating the utility images
  it uses, it can get into a pretty bad state (e.g. reporting failed
  tests as passed)

These instructions are pretty messy and manual, but hopefully writing
this down will be a step toward making the process better!

This configuration has already been applied manually.

README.md

@@ -33,7 +33,53 @@ _[Prow docs](https://github.com/kubernetes/test-infra/tree/master/prow)._
 _[See the community docs](../CONTRIBUTING.md#pull-request-process) for more on
 Prow and the PR process._
 
-### Updating Prow
+### Updating Prow itself
+
+Prow has been installed by taking the
+[starter.yaml](https://github.com/kubernetes/test-infra/blob/master/prow/cluster/starter.yaml)
+and modifying it for our needs.
+
+Updating (e.g. bumping the versions of the images being used) requires:
+
+0. If you are feeling cautious and motivated, manually backup the config values by hand
+   (see [prow.yaml](prow/prow.yaml) to see what values will be changed).
+1. Manually updating the `image` values and applying any other config changes found in the
+   [starter.yaml](https://github.com/kubernetes/test-infra/blob/master/prow/cluster/starter.yaml)
+   to our [prow.yaml](prow/prow.yaml).
+2. Updating the `utility_images` in our [config.yaml](prow/config.yaml) if the version of
+   the `plank` component is changed.
+3. Applying the new configuration with:
+
+   ```yaml
+    # Step 1: Configure kubectl to use the cluster, doesn't have to be via gcloud but gcloud makes it easy
+    gcloud container clusters get-credentials prow --zone us-central1-a --project tekton-releases
+
+    # Step 2: Update Prow itself
+    kubectl apply -f prow/prow.yaml
+
+    # Step 2: Update the configuration used by Prow
+    kubectl create configmap config --from-file=config.yaml=prow/config.yaml --dry-run -o yaml | kubectl replace configmap config -f -
+
+    # Step 3: Remember to configure kubectl to connect to your regular cluster!
+    gcloud container clusters get-credentials ...
+   ```
+4. Verify that the changes are working by opening a PR and **manually looking at the logs of each check**,
+   in case Prow has gotten into a state where failures are being reported as successes.
+
+These values have been removed from the original
+[starter.yaml](https://github.com/kubernetes/test-infra/blob/master/prow/cluster/starter.yaml):
+
+- The `ConfigMap` values `plugins` and `config` because they are generated from
+  [config.yaml](prow/config.yaml) and [plugins.yaml](prow/plugins.yaml)
+- The `Services` which were manually configured with a `ClusterIP` and other routing
+  information (`deck`, `tide`, `hook`)
+- The `Ingress` `ing` - Configuration for this is in [ingress.yaml](prow/ingress.yaml)
+- The `statusreconciler` Deployment, etc. - Created #54 to investigate adding this.
+- The `Role` values give `pod` permissions in the `default` namespace as well as `test-pods` -
+  The intention seems to be that `test-pods` be used to run the pods themselves, but we
+  don't currently have that configured in our [config.yaml](prow/config.yaml).
+
+### Updating Prow configuraiton
 
 TODO(#1) Apply config.yaml changes automatically
 

prow/config.yaml

@@ -6,10 +6,10 @@ plank:
     timeout: 7200000000000 # 2h
     grace_period: 15000000000 # 15s
     utility_images:
-      clonerefs: "gcr.io/k8s-prow/clonerefs@sha256:b62ba1f379ac19c5ec9ee7bcab14d3f0b3c31cea9cdd4bc491e98e2c5f346c07"
-      initupload: "gcr.io/k8s-prow/initupload@sha256:58f89f2aae68f7dc46aaf05c7e8204c4f26b53ec9ce30353d1c27ce44a60d121"
-      entrypoint: "gcr.io/k8s-prow/entrypoint:v20180512-0255926d1"
-      sidecar: "gcr.io/k8s-prow/sidecar@sha256:8807b2565f4d2699920542fcf890878824b1ede4198d7ff46bca53feb064ed44"
+      clonerefs: "gcr.io/k8s-prow/clonerefs:v20190731-e3f7b9853"
+      initupload: "gcr.io/k8s-prow/initupload:v20190731-e3f7b9853"
+      entrypoint: "gcr.io/k8s-prow/entrypoint:v20190731-e3f7b9853"
+      sidecar: "gcr.io/k8s-prow/sidecar:v20190731-e3f7b9853"
     gcs_configuration:
       bucket: "tekton-prow"
       path_strategy: "explicit"