Improve GCS gateway example instruction (#4587)

cmd/gateway-main.go

@@ -113,6 +113,8 @@ ENVIRONMENT VARIABLES:
 
 EXAMPLES:
   1. Start minio gateway server for GCS backend.
+      $ export GOOGLE_APPLICATION_CREDENTIALS=/path/to/credentials.json
+      (Instructions to generate credentials : https://developers.google.com/identity/protocols/application-default-credentials)
       $ export MINIO_ACCESS_KEY=accesskey
       $ export MINIO_SECRET_KEY=secretkey
       $ {{.HelpName}} mygcsprojectid