Suggestions to facilitate contributions on this repository

In the context of Etalab helping the [10% projects](https://github.com/topics/programme10pourcent) to become more collaborative, here are some suggestions:

- [ ] Add a `CONTRIBUTING` section to the `README.md`: explain _who_ can contribute and for what (it is not necessary to provide a dedicated `CONTRIBUTING.md` file).
- [x] Add a `LICENSE` section to the `README.md`.
- [x] Add more tags to the GitHub page.
- [ ] Enhance the GitHub description of the project: the current pointer to an external URL does not allow to quickly understand what this repo is about.