Add the instruction on how to upgrade v0.10 and v0.11 to v0.12 (#265)

* Add the instruction on how to upgrade v0.10 and v0.11 to v0.12

* Remove the section about 0.9 version

README.md

@@ -183,3 +183,7 @@ spec:
   channel: alpha
 EOF
 ```
+
+## Upgrade
+
+Please refer to the [upgrade guide](doc/upgrade_guide.md) for a safe upgrade process.

doc/upgrade_guide.md

@@ -0,0 +1,28 @@
+# Upgrade Guide
+
+This document describes how to upgrade the serving-operator to an expected version.
+
+## Backup precaution
+
+As an administrator, you are recommended to save the content of the custom resource for serving-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 `serving_operator_cr.yaml`:
+
+For the version v0.10.0 or later:
+
+```
+kubectl get KnativeServing <cr-name> -n <namespace> -o=yaml > serving_operator_cr.yaml
+```
+
+Replace `<cr-name>` with the name of your CR, and `<namespace>` with the namespace.
+
+One version of serving-operator installs only one specific version of Knative Serving. With your
+operator successfully upgraded, your Knative Serving is upgraded as well.
+
+## v0.10.0, v0.11.x -> v0.12.0
+
+Both of v0.10 and v0.11 versions are able to upgrade to the version v0.12.0 by running the following command:
+
+```
+kubectl apply -f https://github.com/knative/serving-operator/releases/download/v0.12.0/serving-operator.yaml
+```