Adds instructions to set up default credentials for the Cloud SDK

Author: gguuss

vision/README.md

@@ -15,6 +15,12 @@ To run the sample, first install dependencies:
 
     bundle install
 
+If you haven't already, configure default credentials for using the
+[Cloud SDK](https://cloud.google.com/sdk/):
+
+    gcloud auth login
+    gcloud init
+
 Next, set the configured project by setting the *GOOGLE_CLOUD_PROJECT*
 environment variable to the project name set in the
 [Google Cloud Platform Developer Console](https://console.cloud.google.com):