Start on ReSpec document

[addyosmani]

I've started work on the ReSpec version of the content in this repo. I've ported over all of the initial content and now need to update it to match #14 and #17. Will push to a branch soon.

Some notes as I work through:

• We will be moving forward with a single attribute (name TBD, it could be importance or priority) rather than fetch classes. We will stick (for now) with three values for this initial proposal (high, lowanddefault`). This will require all the examples to be updated.
• We could benefit from a lot more code examples to accompany some of the sample use cases.
• Syntax for Link header.. Link: <https://example.com/other/styles.css>; importance=high; as=style?
• Should the spec discuss how fetch priorities relate to <link rel=preload>? We've had a few questions about how these pieces are connected.
• Providing priority signals for dynamically loaded resources - developers need a way to provide the above signals for resources that are fetched through Javascript, e.g. using the fetch() API. Should we defer specifying out the JS API for this to Fetch Controller / Priorty Control in the Fetch spec?
• Custom fetch groups / importance levels will not be supported in this first version.