📝(documentation) add banner to readme (#970)
Implement suggestions made by @xibe in [#848](https://github.com/suitenumerique/docs/pull/848) and [#849](https://github.com/suitenumerique/docs/pull/849) Signed-off-by: virgile-dev <virgile.deville@beta.gouv.fr>
This commit is contained in: