Autodiscovery lacks documentation around CORS

[joshlevinson]

When working with cross-origin requests, headers are only exposed when whitelisted.
This means autodiscovery breaks unless a Access-Control-Expose-Headers header containing Link is sent.

While this issue is not specific to this library, I think it'd be helpful to document how to make in-browser cross-domain autodiscovery happen.

Happy to submit a documentation PR!

[kadamwhite]

That's a good point, documentation on this issue would be a welcome addition!

[kadamwhite]

@joshlevinson Good to meet you last night! Let me know if you want to pair up on this to knock it out briefly while we're at WCUS.