Create "Web Thing API Protocol Bindings" note about api

[benfrancis]

Note assumptions in core specification:

Assuming don’t need to specify different HTTP and CoAP hrefs. A Thing Description fetched using HTTP can use HTTP URLS and a Thing Description fetched using CoAP can use CoAP URLs. Available alternative protocols can be specified using an Alt-Svc HTTP response header.

Assuming don’t need to specify media types for links because in a JSON encoded Thing Description links are assumed to also be JSON encoded. An alternative encoding can have links to resources in its own encoding, and multiple encodings can share the same URL by using HTTP content negotiation. Available alternative encodings can also be specified in Link rel=alternate response headers.