updated readme to mention ADC

Author: XuechunHou

confmap/provider/googlesecretsprovider/README.md

@@ -55,5 +55,6 @@ service:
 ### Prerequisites
 1. Make sure to enable access to the [Secret Manager API](https://cloud.google.com/secret-manager/docs/accessing-the-api).
 2. Make sure to [add the secret entries to Google Secret Manager](https://cloud.google.com/secret-manager/docs/create-secret-quickstart) before referencing them in the collector configurations. 
-
+3. This Provider interacts with Google Secret Manager using the Secret Manager client library. This library uses [Application Default Credentials (ADC)](https://cloud.google.com/docs/authentication/application-default-credentials) to locate authentication credentials for Secret Manager. Therefore, if you run your collector in a local environment, execute [gcloud auth application-default login](https://cloud.google.com/secret-manager/docs/authentication#client-libs) command to generate the necessary credential file to provide to ADC.
+4. However, if your collector runs on Google Compute Engine (GCE) or Google Kubernetes Engine (GKE), running `gcloud auth application-default login` is optional. This is because ADC can retrieve credentials via [the metadata server](https://cloud.google.com/docs/authentication/application-default-credentials#order). However, ensure that your GKE or GCE instance [has enabled the cloud-platform OAuth scope](https://cloud.google.com/secret-manager/docs/accessing-the-api#oauth-scopes). Additionally, verify that the Service Account attached to the GCE or GKE instance has been granted at least the [roles/secretmanager.secretAccessor](https://cloud.google.com/secret-manager/docs/access-control#secret-manager-roles) IAM role to access secret entries in Google Secret Manager.