Add instructions for v1alpha1 to v1beta1 promotion

Author: maqiuyujoyce

README.NewResourceFromTerraform.md

@@ -236,6 +236,14 @@ ServiceMappings file. Add the `ResourceConfig` for your resource:
 
 1.  Fill out the `resourceAvailableInAssetInventory`. Set to false.
 
+1.  If you are adding a resource that is already supported in v1alpha1, specify
+    the following fields
+    ([example](https://github.com/GoogleCloudPlatform/k8s-config-connector/blob/8ba13a0c1e0563e0e6f9a9f6eeb8a693f9c73acf/config/servicemappings/certificatemanager.yaml#L30-L31)):
+    1.  Set `v1alpha1ToV1beta1` to `true`.
+    1.  Set `storageVersion` to `v1alpha1`.
+
+1.  Ensure the `autoGenerated` field is unset.
+
 1.  Add `hierarchicalReferences` if they exist. Determine if the corresonding
     Terraform resource exposes one or more parent resources through top-level
     fields. These fields can have names like `org_id`, `parent_org_id`,