Add installation instructions for OpenShift

This commit documents the steps required to install Chains on OpenShift.

README.md

@@ -56,6 +56,44 @@ NAME                                       READY   STATUS      RESTARTS   AGE
 tekton-chains-controller-c4f7c57c4-nrjb2   1/1     Running     0          160m
 ```
 
+#### Installation on OpenShift
+Prerequisite: you'll need [Tekton Pipelines](https://github.com/tektoncd/pipeline/blob/main/docs/install.md#installing-tekton-pipelines-on-openshift) installed on your OpenShift cluster before you install Chains.
+
+To install Tekton Chains on OpenShift, you must first apply the `anyuid` security
+context constraint to the `tekton-chains-controller` service account. This is required to run the controller pod.
+See
+[Security Context Constraints](https://docs.openshift.com/container-platform/4.3/authentication/managing-security-context-constraints.html)
+for more information.
+
+1. Log on as a user with `cluster-admin` privileges. The following example
+   uses the default `system:admin` user:
+
+   ```bash
+   oc login -u system:admin
+   ```
+
+1. Set up the namespace (project) and configure the service account:
+
+   ```bash
+   oc new-project tekton-chains
+   oc adm policy add-scc-to-user anyuid -z tekton-chains-controller
+   ```
+
+1. Install Tekton Chains:
+
+   ```bash
+   oc apply --filename https://storage.googleapis.com/tekton-releases/chains/latest/release.yaml
+   ```
+   See the
+   [OpenShift CLI documentation](https://docs.openshift.com/container-platform/4.3/cli_reference/openshift_cli/getting-started-cli.html)
+   for more information on the `oc` command.
+
+1. Monitor the installation using the following command until all components show a `Running` status:
+
+   ```bash
+   oc get pods --namespace tekton-chains --watch
+   ```
+
 ### Setup
 
 To finish setting up Chains, please complete the following steps: