[client-hints] new v1 spec draft and src-N interop

The goal of Client Hints is to automate (where possible) the negotiation of
optimal resolution and size of delivered image resources.

- For a quick intro: https://github.com/igrigorik/http-client-hints
- New v1 draft:
https://github.com/igrigorik/http-client-hints/blob/master/draft-grigorik-http-client-hints-01.txt
- For server folks (or if you want to see it in action):
https://github.com/igrigorik/http-client-hints/tree/master/server

With that in mind, a quick overview of differences from previous draft:

   - Hints are sent in different headers (e.g. CH-DPR: 2.0) - this allows
   better interop with existing caches [1]
   - New "server confirmation" header (e.g. "DPR: 1.5") allows server to
   indicate the DPR of served resource, which is then used to adjust intrinsic
   size calculations on the client. [2]
   - "Device width" has been replaced with "Resource display width" (in
   DIPs) [3]

Also, I've added a section on interop with src-N (see [4]) to demonstrate
how CH and src-N can work together to simplify the various RICG use cases.
That said, note that the choice of using/not-using either src-N and/or CH
is completely up to you. You can combine both and get the benefit of
automation of CH + art-direction of src-N, or, you can use src-N for
everything, or, you can just use CH to automate resolution switching (viar
CH-DPR hint) and resizing (via CH-RW hint).

Finally, we have a CH prototype in Chrome that you can play with, check out
the instructions here:
https://github.com/igrigorik/http-client-hints/tree/master/server#client-hints-clients

Feedback, suggestions, questions, etc.. all welcome!

ig

[1] https://github.com/igrigorik/http-client-hints/issues/14
[2] https://github.com/igrigorik/http-client-hints/issues/13
[3]
https://github.com/igrigorik/http-client-hints/blob/master/draft-grigorik-http-client-hints-01.txt#L239
[4] https://github.com/igrigorik/http-client-hints#interaction-with-src-n