feat: added CONTRIBUTING.md for OKDP contribution workflow

[jpmewenemesse]

Purpose

This PR covers the contribution workflow for OKDP repositories:

1. issue reporting
2. fork-based vs direct branch workflow depending on repo visibility
3. commit message conventions
4. PR review process

[lioneloh]

pas encore public, donc a supprimer

[Pierrotws]

non-explicit action described here. Could be misleading

[Pierrotws]

What about AI usage, is it allowed ?
If not : what are consequences of hidden usage ? should set a checkbox to confirm at PR that AI has not been used.
Else : should the AI set as co-author ?
Example, in end of commit message

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

[jpmewenemesse]

@Pierrotws I think your question is worth adressing. But for me,

• Currently public sector does not officially approve AI usage like Claude or Gemini.... at least at DGFiP, this is not allowed.

More broadly, I dont think enforcement at contribution level is necessary or appropriate. AI assisted is another productivity tool like VSCode or Linter and contributors remain fully responsible for what they submit.

• Am not sure the CLA currently in progress integrate an AI as co-author or responsability is fully human (cc @lioneloh ).

Where I think there is more interesting conversation is at the project level. Maybe OKDP can think of integrating AI capabilities (AGENTS, Skills, automated review assistance...) like FluxCD AGENTS.md or agents skills rather than governing how individual contributors use their own tools? It is worth discussing at next TOSIT meeting.