Update installation.md (#946)

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**Which issue(s) this PR fixes**:

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This PR was merged by Kapetanios.

docs/content/en/docs/operator-manual/control-plane/installation.md

@@ -142,3 +142,12 @@ This part provides guidance for a production hardened deployment of the control
     Those key and cert can be configured via [`secret.internalTLSKey.data`](https://github.com/pipe-cd/manifests/blob/master/manifests/pipecd/values.yaml#L83) and [`secret.internalTLSCert.data`](https://github.com/pipe-cd/manifests/blob/master/manifests/pipecd/values.yaml#L86).
 
     To enable internal tls connection, please set the `gateway.internalTLS.enabled` parameter to be `true`.
+
+    Otherwise, the `cloud.google.com/app-protocols` annotation is also should be configured as the following:
+
+    ``` yaml
+    service:
+      port: 443
+      annotations:
+        cloud.google.com/app-protocols: '{"service":"HTTP2"}'
+    ```