Docs updates

README.md

@@ -65,9 +65,16 @@ instead of `from_function()`;
 
 Looks for credentials in the following places, preferring the first location found:
 - A JSON file whose path is specified by the GOOGLE_APPLICATION_CREDENTIALS environment variable.
-- A JSON file in a location known to the gcloud command-line tool.
+- A JSON file in a location known to the gcloud command-line tool using `gcloud auth application-default login`.
 - On Google Compute Engine, it fetches credentials from the metadata server.
 
+### Local development
+Don't confuse `gcloud auth login` with `gcloud auth application-default login` for local development,
+since the first authorize only `gcloud` tool to access the Cloud Platform.
+
+The latter obtains user access credentials via a web flow and puts them in the well-known location for Application Default Credentials (ADC).
+This command is useful when you are developing code that would normally use a service account but need to run the code in a local development environment where it's easier to provide user credentials.
+So to work for local development you need to use `gcloud auth application-default login`.
 
 ## Fork
 The library is a fork of [mechiru/googapis](https://github.com/mechiru/googapis) and [mechiru/gouth](https://github.com/mechiru/gouth) to keep up with