Add Documentation on Fast Fetch expected ad response

This is a documentation update request.

## What's the issue?

Fast fetch network implementation is unclear.
Please see this request on stackoverflow with no answer:
https://stackoverflow.com/questions/62267848/getting-started-with-amp-and-rtc-as-a-vendor

### Tips on Rendering Bugs

When trying to implement an AMP RTC integration, we find the need to implement a Fast fetch adserver endpoint. Can you please develop this point ? Server to server ad serving is standardized using IAB's oRTB specifications. Is it what's expected here ?

Informations on Ad server requirements can be found here
https://github.com/ampproject/amphtml/blob/master/ads/google/a4a/docs/Network-Impl-Guide.md#ad-server-requirements

Request, headers SSL ... but nothing on the expected response format.