Strengthening language for subdomain configuration

README.md

@@ -132,7 +132,9 @@ Display Farsi version of Javascript and CSS
 
 ## Configuration - Sandboxing ##
 
-For improved security, you may want to have cached content served from a separate domain. Here is a sample configuration where the site is running at www.amber.com, while cached content is served from sandbox.amber.com.
+For improved security, you must next configure Amber to serve cached content from a separate domain. We recommend that you establish a subdomain for this purpose.
+
+Here is a sample configuration where the site is running at www.amber.com, while cached content is served from sandbox.amber.com.
 
     http {
         [...]