Update contributing

Author: garronej

CONTRIBUTING.md

@@ -5,6 +5,7 @@ Thank you so much to anyone that would!
 
 To get you started you can check out [the `<Alert />` component](https://react-dsfr-components.etalab.studio/?path=/docs/components-alert--default).
 
+-   Here is it's definition from the SIG: [systeme-de-design.gouv.fr/elements-d-interface/composants/alerte](https://www.systeme-de-design.gouv.fr/elements-d-interface/composants/alerte)
 -   Here is its implementation [src/Alert.tsx](https://github.com/codegouvfr/react-dsfr/blob/main/src/Alert.tsx)
 -   Here is the file that generates its documentation: [stories/Alert.stories.tsx](https://github.com/codegouvfr/react-dsfr/blob/main/stories/Alert.stories.tsx)