Add contributing documentation

CONTRIBUTING.md

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+# kouto swiss: contributing
+
+You want to contribute to **kouto swiss* ? It's great! I'm really glad to see you want it too.
+
+This little document will help you to contribute to the project easily by presenting you the tools I made to help you.
+
+There's two way to contribute to **kouto swiss**: add new functionalities or bugfix the stylus libraries, or, if you don't want/can't write stylus code, you can help me rewrite the texte of the documentation, since my english, is, I know, very bad.
+
+### Setup the project for development
+
+Simply fork & clone the repository, and launch the command `npm install` from inside the **kouto swiss** folder.
+
+## Contribute to the lib
+
+Each functionality of the **kouto swiss** lib has 3 parts: 
+
+1. the unit tests
+2. the functionality's code
+3. the functionality's documentation
+
+### Unit Tests
+
+If you look at the folder `test/cases`, you will see many tests here. Each test consist of two files: a `.styl` file, and a `.css` file. As you can guess, The stylus file is compiled using the lib and should be equal to the result in css file.
+
+Each time you want to add/modify a functionality, be sure to begin your work by adding the corresponding tests.
+
+You can run the test by launching this command: `grunt test`.
+
+### Functionality's code
+
+All the functionalities of **kouto swiss** are stored in the `lib/` folder of the repository. Simply add your new functionality's file to the corresponding folder, and don't forget to import it into the `index.styl` file of the folder.
+
+After each addition and/or modification, be sure **all** the tests are running well.
+
+### Functionality's documentation
+
+The documentations for the funtionnalities of **kouto swiss** are store in the `_docs` folder. These are written in `markdown` format and follow some simple structure you can easely understand by reading the existent doc files.
+
+Don't forget to add your new functionnality to the `_doc/index.json` file, which is used to generate the documentation page.
+
+You can preview the **kouto swiss** website locally with the following command: `grunt preview`, which allows you to see the site on [localhost:5555](http://localhost:5555).
+
+## Contribute to the documentation website
+
+The process is much simpler here: the documentation website's files are stored in the `_docs` folder.
+
+Before modifying anything, launch the following command: `grunt work`, which will compile all the files and launch a web server for previewing the site on [localhost:5555](http://localhost:5555). This preview has a *livereload* feature included if you want to use it.
+
+Simply modify/correct the files and the task will recompile your changes.
+
+* * *
+
+Thanks in advance for helping me making **kouto swiss** a great and useful tool for all the stylus users! :)