docs: explain dev-deps install for NPM 3

README.md

@@ -132,21 +132,29 @@ We use [Gulp](http://gulpjs.com/) to build the project, so if you want to re-bui
 
 After gulp is installed, and your CLI is pointed to your work directory, first install the dependencies:
 
+**with NPM 2.x.x**
+
 ```
-     npm install
+npm install
 ```
 
+**with NPM 3.x.x** (resolve dependencies for `node-spritesheet` before this module's ones)
+
+```
+npm install grunt grunt-contrib-coffee grunt-contrib-clean
+npm install
+```
+
+then be sure that you have *[ImageMagick](http://www.imagemagick.org/script/binary-releases.php)* installed for building spritesheet.
 
 then, you can run the `gulp build` task to build the project:
 
 
 ```
-     gulp build
+gulp build
 ```
 
 
-
-
 **What the build task does?**
 
 First, it copies GIF files from the `src` folder, and pastes them to the `dist` folder.