Refactor specifications to README

Summary:
Context: SWH, Inria and the CCSD (in charge of the HAL's archive) are working on pushing
HAL's source code deposit into SWH with SWORD protocol.

This specs were edited by ardumont and myself on the notepad
available on: https://pad.riseup.net/p/t4mJsj9RB0OQ

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D220