Add exercice chain.js
Add exercice chain.js

Karimjy/ecvd-js,maechlerludivine/ecvd-js,morgangiraud/ecvd-js,tonylucas/ecvd-js,guillaumebarranco/ecvd-js,Rouksana/ecvd-js,maechlerludivine/ecvd-js,guillaumebarranco/ecvd-js,maechlerludivine/ecvd-js,tonylucas/ecvd-js,Rouksana/ecvd-js,Karimjy/ecvd-js,morgangiraud/ecvd-js,Rouksana/ecvd-js,morgangiraud/ecvd-js