[Documentation] Guidance for AutoRest config README contents

Many parts of our process depend on the `readme.md` file for a service in azure-rest-api-specs, but I am not aware of any documentation on the required structure or content of this file. When the "avocado" check for a PR fails because of incorrect readme content or structure, service teams are left to guess what should be done fix.

We need to clearly document the required structure and contents of `readme.md` and put this in an easily discoverable location.

cc: @ccbarragan @josefree @konrad-jamrozik