Add listing CDN containers to Object Store user guide

[ycombinator]

This should include:

• One or more sections in https://github.com/rackspace/php-opencloud/blob/master/docs/userguide/ObjectStore/USERGUIDE.md
• Corresponding code samples in https://github.com/rackspace/php-opencloud/tree/working/samples/ObjectStore

See jamiehannaford@a9f8bdd for reference.

[jamiehannaford]

This has documentation now, so closing