Explain the "localized content" story

This becomes a subsection or subpage of the (latest version of) the Delivery API page.

Maybe link to AdminCentral and public locales

See MGNLREST-153 for all the rules which are applied to determine the local.
Explain ho to use it with a little example.

Explain the JSON structure - also how it is now different from what we got in response before.

Stephen Nguyen knows all the details.