recommend restricting to HTTP referrer

Author: steren

README.md

@@ -16,7 +16,7 @@ Here's an introductory video:
 ## Prerequisites
 
 1. You need a [Google Cloud project](https://console.cloud.google.com).
-1. [Enable the Stackdriver Error Reporting API](https://console.cloud.google.com/apis/api/clouderrorreporting.googleapis.com/overview) for your project.
+1. [Enable the Stackdriver Error Reporting API](https://console.cloud.google.com/apis/api/clouderrorreporting.googleapis.com/overview) for your project. We highly recommend to restrict the usage of the key to your website URL only using an 'HTTP referrer' restriction.
 
 ## Quickstart