Add PEP compliant Documentation if JSON API PEP Accepted

[cooperlees]

Proposing some slight modifications to the JSON API to allow custom indexes to implement. e.g. devpi

Issue here: pypa/packaging-problems#367

This is a reminder to update the spec here if accepted: https://github.com/pypa/packaging.python.org/tree/master/source/specifications