Update README.md

@@ -13,7 +13,22 @@ You can see more resources about ARCS at [its website](http://arcs.ibisc.fr)
 
 How to build
 ------------
+The first thing is to clone this repository.
+
+ARCS.js is using three tools in order to be built: 
+ - `npm`, the [Node.js](https://nodejs.org) package manager, 
+ - `grunt`, a [task runner](http://gruntjs.com/) used as a build tool,  
+ - `bower`, a [package manager](https://bower.io/) in order to ship ARCS.js dependencies.
+
+We suppose that npm is already installed. Then, you will have to setup your 
+development environment by typing these commands, 
+the last one being executed in the local sources repository:
+
+`npm install -g bower`
+
+`npm install -g grunt-cli`
+
+`npm install`
+
 
-npm install
 
-Will install all node dependencies needed to build and develop the package.