doc(readme): nécessite Debian Bullseye pour Node.js (fix #29)

{{cookiecutter.project_slug}}/README.md

@@ -158,8 +158,8 @@ The assets - e.g. CSS, JavaScript, images, fonts - are generated using a
 - Built-in development server
 - Compression for production builds
 
-You will need to install [npm](https://www.npmjs.com/). It is included in Debian
-since Buster - and also in the `stretch-backports` repository.
+You will need to have Node.js >= 12.13.0 and npm installed. Note that this is
+included in Debian since Bullseye through `nodejs` and `npm` packages.
 
 Start by installing the application dependencies - which are defined in
 `package.json` - by running: `npm install`.