Add Potentiel in the usecase list (#215)

Author: spontoreau

README.md

@@ -130,6 +130,7 @@ A few projects that use `@codegouvfr/react-dsfr`.
 -   https://github.com/etalab/sill-web
 -   https://github.com/inclusion-numerique/mediature
 -   https://territoiresentransitions.fr (maybe)
+-   https://potentiel.beta.gouv.fr
 
 Keep in mind that the project has been released recently so it's only natural
 that there is only a few projects in production using it.