Provide explicit examples for storing plugin credential files

[ekinanp]

When I do

docs gcp
This is the GCP plugin root. It follows https://cloud.google.com/docs/authentication/production
to find your credentials:

* it will try GOOGLE_APPLICATION_CREDENTIALS as a service account file
* otherwise, it will use your credentials in $HOME/.config/gcloud/application_default_credentials.json.

The simplest way to set this up is with
...

The GOOGLE_APPLICATION_CREDENTIALS thing should be something like

* it will try GOOGLE_APPLICATION_CREDENTIALS as a service account file. You can set this to something like GOOGLE_APPLICATION_CREDENTIALS=~/.gcp_credentials.json