README: add installation instruction

Author: Bluzzi

README.md

@@ -4,8 +4,29 @@ A simple template for website created with Express JS.
 
 This template use the Express package from Node.js and offers to start working with an organized project wich contains everything you need without useless assets.
 
-## Actual content
-
+## Installation 
+Do this commands by replacing ``projectName`` with the name of your project :
+```sh
+npx degit https://github.com/Leepo-Team/Express-Template projectName
+cd projectName
+```
+
+Then install the packages :
+```sh
+npm install
+```
+
+You can then launch the site in development mode with the command :
+```sh
+npm run dev
+```
+
+Or in production mode :
+```sh
+npm run start
+```
+
+## Organization and additional content
 - Support for cookies and sessions.
 - "public" folder wich will contain everything the visitor has to download: pictures, fonts, scripts, stylesheets...
 - Simple routes system.