Add a french version of the README

[bzg]

I suggest having README.fr.md on top of the default README.md, providing at the least the basic information on what this library does and how to use it.

Maintaining translations is hard, so I would not make it mandatory to have the same content in both README.fr.md and README.md: if each file is useful, that's good enough.

[garronej]

D'accord oui, je m'en charge au jour d'hui

[garronej]

Done: https://github.com/codegouvfr/react-dsfr/blob/main/README.fr.md

[bzg]

Done: https://github.com/codegouvfr/react-dsfr/blob/main/README.fr.md

Top, merci beaucoup !