Add publiccode.yml

[bzg]

https://github.com/publiccodeyml/publiccode.yml is a metadata schema to describe software projects.

https://code.gouv.fr/#/repos allows to filter repositories by whether they contain a publiccode.yml file or not. The Free Software unit (Etalab/DINUM) encourages administrations to add publiccode.yml files to their repositories with a high reusability potential, which is the case for Onyxia: it helps identifying reuses of the repo since they are manually added to the publiccode.yml file.

@hjonin at Etalab will help administrations discover and use PublicCode - we have this website to help understand the schema, don't hesite to ping us here.