Add Article for Cache API

[ashleymichal]

This is already started in #42 . Necessary additions:

Document the API surface (objects, methods, properties implemented). Use https://wiki.cfops.it/display/EW/Runtime+API+Doc+Questions for reference, link to mdn where appropriate.

Important to note:

• put will throw an error if the request method is not GET
• match will simply return undefined if the request method is not GET
• put will also throw an error on a response with status code 206.

This issue is part of #11 and related to #44

[harrishancock]

One last cause of put() throwing is trying to store a response with a Vary: * header. Our cache doesn't actually honor the Vary header AFAIK, though -- we just throw because it's required by the spec.

[ashleymichal]

closed by #42