chore(kgo): add notes about upgrading (#1158)

charts/gateway-operator/README.md

@@ -11,6 +11,10 @@ helm repo update
 helm install kgo kong/gateway-operator -n kong-system --create-namespace
 ```
 
+## Minimal supported versions
+
+This chart supports operator versions starting from 1.2.0.
+
 ### CRD management
 
 Helm 3 introduced a simplified CRD management method that is safer than what was

charts/gateway-operator/UPGRADE.md

@@ -14,6 +14,16 @@ Note that not all versions contain breaking changes. If a version is not
 present in the table of contents, it requires no version-specific changes when
 upgrading from a previous version.
 
+## Updating operator version
+
+The operator version is following [SemVer][semver].
+This means that users should not expect breaking changes without a major version change.
+
+Any changes requiring manual user action will be called out in operator [release notes][kgo_release_notes].
+
+[semver]: https://semver.org/
+[kgo_release_notes]: https://github.com/Kong/gateway-operator/blob/main/CHANGELOG.md
+
 ## Updates to CRDs
 
 Helm installs CRDs at initial install but [does not update them after][hip0011].