Add a user guide for querying the API from the browser

[kadamwhite]

There is some good content in #51 and http://wp-api.org/node-wpapi/guides/2016/08/12/authenticating-with-cookies.html that could be adapted. We should have a guide that goes beyond the existing cookie authentication callout and shows how to create, enqueue, and localize a script that does a basic authenticated REST API interaction, possibly using jQuery.