Add CONTRIBUTING.md

[bzg]

Are contributions welcome? What should contributors know and do? Sign a developers certificate of origin (DCO)? A copyright license agreement (CLA)?

A CONTRIBUTING.md file could explain all this.

If contributions are expected only from other FR public agents, a DCO seems enough.

If contributions are also welcome from anyone, then some thinking must be done about whether a CLA would help or not.

For an example of CONTRIBUTING.md file with a CLA, see https://github.com/GouvernementFR/dsfr/blob/main/CONTRIBUTING.md.

Copying @hjonin on this as this is something Etalab's Free Software unit can help about.

[bzg]

Iff https://github.com/inseeFrLab/onyxia-web replaces https://github.com/inseeFrLab/onyxia as the main repo, the CONTRIBUTING.md in https://github.com/inseeFrLab/onyxia-web is okay, the URL it points to just need to be proofread to make sure pointers are up to date.