Add custom payload test

docs/ custom-payload-testing.md

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+# Custom payload testing
+
+OpenShift v4 uses a release payload image to configure the cluster. Sometimes we
+need to test custom payload before we submit a patch to upstream. This
+section describes how to do this.
+
+1. Build the required operator component and host it to the external public
+repository.
+
+1. Pull release payload you want to use as a base to test your changes. An
+example:
+```
+export REGISTRY_AUTH_FILE=/home/$USER/.rh/pull-secret.txt
+podman pull quay.io/openshift-release-dev/ocp-release-nightly@sha256:ab5022516a948e40190e4ce5729737780b96c96d2cf4d3fc665105b32d751d20
+```
+
+1. Extract required files
+```
+podman run  -it --entrypoint "/bin/bash" -v /tmp/cvo:/tmp/cvo:z \
+--rm quay.io/openshift-release-dev/ocp-release-nightly@sha256:ab5022516a948e40190e4ce5729737780b96c96d2cf4d3fc665105b32d751d20
+ cat release-manifests/0000_50_cloud-credential-operator_05_deployment.yaml > /
+ tmp/cvo/0000_50_cloud-credential-operator_05_deployment.yaml
+```
+
+1. Apply changes you need and build image using example `Dockerfile`
+```
+FROM quay.io/openshift-release-dev/ocp-release-nightly@sha256:ab5022516a948e40190e4ce5729737780b96c96d2cf4d3fc665105b32d751d20
+ADD 0000_50_cloud-credential-operator_05_deployment.yaml /release-manifests/0000_50_cloud-credential-operator_05_deployment.yaml
+ADD image-references /release-manifests/image-references
+```
+
+1. Publish images to personal repository and update code to use new payload
+```
+podman build -t quay.io/$USER/release-payload ./
+podman push quay.io/$USER/release-payload
+```