Document GOOGLE_APPLICATION_CREDENTIALS support.
Document GOOGLE_APPLICATION_CREDENTIALS support.

We've supported GOOGLE_APPLICATION_CREDENTIALS as an environment
variable (it comes free with our OAuth2 client) but it has never been
documented. Documenting it now to resolve #12626.

drebes/terraform-provider-google,terraform-providers/terraform-provider-google,rosbo/terraform-provider-google,hashicorp/terraform-provider-google-beta,ubschmidt2/terraform-provider-google,rosbo/terraform-provider-google,drebes/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,selmanj/terraform-provider-google,hashicorp/terraform-provider-google,nickithewatt/terraform-provider-google,selmanj/terraform-provider-google,drebes/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,rosbo/terraform-provider-google,hashicorp/terraform-provider-google,ubschmidt2/terraform-provider-google,nickithewatt/terraform-provider-google,hashicorp/terraform-provider-google,nickithewatt/terraform-provider-google,hashicorp/terraform-provider-google-beta,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google-beta,hashicorp/terraform-provider-google-beta,rosbo/terraform-provider-google,selmanj/terraform-provider-google,drebes/terraform-provider-google