Provide a sample shortcode to allow testing

[benlk]

We got a support ticket from a user who doesn't have access to the list of active plugins, who wanted to know whether it was active.

• assemble a demo shortcode
• add the demo shortcode to the plugin's docs or FAQ in the readme, under a heading something like "I don't have access to my site's plugins page. How can I tell if this shortcode is active?" with a response along the lines of "create a draft post with this shortcode and preview it; you should see something like this:"

[benlk]

Taken care of in #34, all over the place.