diff --git a/LISEZMOI.md b/LISEZMOI.md
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+SeaCMS Default Theme
+==================
+
+ - [English](README.md)
+
+Ce paquet est le thème principal de SeaCMS.
+
+**Attention : le nom SeaCMS va changer car il semble déjà utilisé dans d'autres pays**
+
+## Auteurs
+
+  - Jérémy Dufraisse (2022-2023)
+  - Thatoo (2022-2023)
+  - Marion Bouder (2022-2023)
+  - Association Defis (2022-2023)
+
+## Documentation
+
+Quelques brides de documentation sont disponibles dans le fichier [doc/en/dev.md](doc/en/dev.md)
+
+## Licence et garanties
+
+Voir le fichier [LICENCE](./LICENCE) (et sa traduction française [LICENCE_FR](./LICENCE_FR))
+
+**IMPORTANT**: Le fichier LICENCE file ne couvre pas le contenu des dossiers `vendor`. Voir le fichier LICENCE fournis dans chaque sous-dossier concerné par la bibliothèque importée.
+
diff --git a/README.md b/README.md
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 SeaCMS Default Theme
 ==================
 
- - [Français](#Fran%C3%A7ais)
- - [English](#English)
-
-## Français
-
-Ce paquet est le thème principal de SeaCMS.
-
-**Attention : le nom SeaCMS va changer car il semble déjà utilisé dans d'autres pays**
-
-### Auteurs
-
-  - Jérémy Dufraisse (2022-2023)
-  - Thatoo (2022-2023)
-  - Marion Bouder (2022-2023)
-  - Association Defis (2022-2023)
-
-### Licence et garanties
-
-Voir le fichier [LICENCE](./LICENCE) (et sa traduction française [LICENCE_FR](./LICENCE_FR))
-
-**IMPORTANT**: Le fichier LICENCE file ne couvre pas le contenu des dossiers `vendor`. Voir le fichier LICENCE fournis dans chaque sous-dossier concerné par la bibliothèque importée.
-
-## English
+ - [Français](LISEZMOI.md)
 
 This package is the main theme of SeaCMS.
 
 **Warning : the name SeaCMS will change because it seems already uses in other countries**
 
-### Authors
+## Authors
 
   - Jérémy Dufraisse (2022-2023)
   - Thatoo (2022-2023)
   - Marion Bouder (2022-2023)
   - Association Defis (2022-2023)
 
-### Licence and warranty
+## Documentation
+
+Some documentation is available into [doc/en/dev.md](doc/en/dev.md)
+
+## Licence and warranty
 
 See [LICENCE](./LICENCE) file (and french translation [LICENCE_FR](./LICENCE_FR))
 
diff --git a/doc/en/dev.md b/doc/en/dev.md
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+# Documentation on development
+
+## Using `yarn`
+
+[`yarn`](https://yarnpkg.com) is a package manager running on [`nodejs`](https://nodejs.org). It is veru useful to manage `javascript` or `css` projects.
+
+### Adding a new package
+
+1. find the name of the wanted package on the website <https://www.npmjs.com>
+2. on command line interface, on the root folder of this project, type `yarn add --dev name-of-the-package`
+3. Identify the needed files into folder `node_modules/name-of-the-package` ; most of time, files are into folder `node_modules/name-of-the-package/dist`
+4. modifify the file `js/extract-files-from-node-modules` to a new line for each file to import in the theme :
+  - `copySync('node_modules/name-of-the-package/dist/js/file.min.js','js/vendor/name-of-the-package/file.min.js',{ overwrite: true })` for `javascript` file
+  - `copySync('node_modules/name-of-the-package/dist/css/file.min.css','css/vendor/name-of-the-package/file.min.css',{ overwrite: true })` for `css` file
+  - do not forget `LICENCE` file of the package, `copySync('node_modules/name-of-the-package/dist/LICENCE','css/vendor/name-of-the-package/LICENCE',{ overwrite: true })` for `css` (and for `javascript`)
+5. type `yarn install` in commpand line interface to start this script
+6. add a new line in concerned `twig` file (example : `includes/_head.twig`) to use the new file (example : `<link href="{{ theme_url }}/css/vendor/name-of-the-package/file.min.css" rel="stylesheet" type="text/css" />`)
+
+### Removing a new package
+
+1. find the name of the wanted package into file `package.json`
+2. on command line interface, on the root folder of this project, type `yarn remove name-of-the-package`
+3. Identify the associated files into folders `js/vendor` and `css/vendor` and delete them
+4. modifify the file `js/extract-files-from-node-modules` to remove lines related to this package
+5. remove line in concerned `twig` file (example : `includes/_head.twig`) that uses the new file (example : `<link href="{{ theme_url }}/css/vendor/name-of-the-package/file.min.css" rel="stylesheet" type="text/css" />`)
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