Clarifying pre-requisite

This code snippet on getting the public URL of an object is much farther below the snippet where the container is CDN-enabled. I know at least one customer who tried this code snippet without the CDN-enabling code snippet first and failed as a result. 

This PR adds a single sentence that notes that CDN-enabling a container is a pre-requisite to getting the public URL of an object. It links to the code snippet on CDN-enabling a container.