doc: frontend: document that X-Ms-Identity-URL HTTP header is now required during creation

Author: miguelsorianod

frontend/README.md

@@ -77,12 +77,17 @@ curl -X GET "localhost:8443/subscriptions/00000000-0000-0000-0000-000000000000/r
 ```
 
 Create or Update a HcpOpenShiftClusterResource
+
 ```bash
 curl -X PUT "localhost:8443/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/dev-test-rg/providers/Microsoft.RedHatOpenShift/hcpOpenShiftClusters/dev-test-cluster?api-version=2024-06-10-preview" \
   -H "X-Ms-Arm-Resource-System-Data: {\"createdBy\": \"aro-hcp-local-testing\", \"createdByType\": \"User\", \"createdAt\": \"2024-06-06T19:26:56+00:00\"}" \
+  -H "X-Ms-Identity-Url": https://dummyhost.identity.azure.net" \
   --json @cluster.json
 ```
 
+You will notice that the request contains a `X-Ms-Identity-Url` with the value `https://dummyhost.identity.azure.net`. Setting the `X-Ms-Identity-Url` HTTP header when interacting directly
+with the Frontend is required. However, for the environments where a real managed identities data plane does not exist the value can be any arbitrary/dummy HTTPS URL that ends in `identity.azure.net`.
+
 Delete a HcpOpenShiftClusterResource
 ```bash
 curl -X DELETE "localhost:8443/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/dev-test-rg/providers/Microsoft.RedHatOpenShift/hcpOpenShiftClusters/dev-test-cluster?api-version=2024-06-10-preview"