Playground should provide CORS advice

[dlongley]

The playground should have a "help" section that either shows up in a context-sensitive way (once a possible CORS error is detected) or is just available via a button/whatever. This should indicate which CORS headers need to be set in order for remote documents or contexts to be loaded by the playground.

Note that the HTTP Link header is a non-simple header and, if it is to be readable by the playground, then "Access-Control-Expose-Headers: Link" must be set on the HTTP response for data loaded with this header.

[dlongley]

Provide an example .htaccess file:

Header set Access-Control-Allow-Origin *
AddType application/ld+json .jsonld