Add the guidance on upgrade (#80)

* Add the guidance on upgrade

* Add the command to save the CR

README.md

@@ -105,3 +105,7 @@ output of the above `ko publish` command.
 The image should match what's in [config/operator.yaml](config/operator.yaml)
 and the `$VERSION` should match [version.go](version/version.go) and correspond
 to the contents of [config/](config/).
+
+## Upgrade
+
+Please refer to the [upgrade guide](doc/upgrade_guide.md) for a safe upgrade process.

doc/upgrade_guide.md

@@ -0,0 +1,49 @@
+# Upgrade Guide
+
+This document describes how to upgrade the eventing-operator to an expected version.
+
+## Backup precaution
+
+As an administrator, you are recommended to save the content of the custom resource for eventing-operator
+before upgrading your operator. Make sure that you know the name and the namespace of your CR, and use the
+following command to save the CR in a file called `eventing_operator_cr.yaml`:
+
+For the version v0.12.0 or later:
+
+```
+kubectl get KnativeEventing <cr-name> -n <namespace> -o=yaml > eventing_operator_cr.yaml
+```
+
+For the version v0.11.0 or earlier:
+
+```
+kubectl get Eventing <cr-name> -n <namespace> -o=yaml > eventing_operator_cr.yaml
+```
+
+Replace `<cr-name>` with the name of your CR, and `<namespace>` with the namespace.
+
+One version of eventing-operator installs only one specific version of Knative Eventing. With your
+operator successfully upgraded, your Knative Eventing is upgraded as well.
+
+## v0.11.0 -> v0.12.0
+
+The Kind name of the custom resource has been changed from `Eventing` to `KnativeEventing`. The version
+v0.12.0 is able to recognize both of the CRs.
+
+Update the eventing-operator to the version v0.12.0 with the command:
+
+```
+kubectl apply -f https://github.com/knative/eventing-operator/releases/download/v0.12.0/eventing-operator.yaml
+```
+
+## v0.10.0 -> v0.11.0
+
+If your existing operator is at the version v0.10.0, you cannot directly upgrade it to v0.12.0 or
+later. We only support upgrade from v0.10.0 to v0.11.0. Existing issues about directly upgrading from v0.10.0
+to v0.12.0 has been report at [Bug 54](https://github.com/knative/eventing-operator/issues/54).
+
+Update the eventing-operator deployment to the version v0.11.0 with the command:
+
+```
+kubectl apply -f https://github.com/knative/eventing-operator/releases/download/v0.11.0/eventing-operator.yaml
+```