Add grid description to README

README.md

@@ -35,3 +35,21 @@ Boilerplate for [`Charcoal`](#http://charcoal.locomotive.ca) projects by Locomot
   classes (e.g. `.hidden {}`).
 
 *From [ITCSS](https://twitter.com/itcss_io)*
+
+### Grid
+
+We are using a simple inline-block grid system.
+
+**Simple usage**
+
+Insert a `.grid` block and add `.grid__item` elements inside it. 
+
+No rows that contain floats, no twelve columns system; just the number of items you want, with fractions like names *(ex: `.third`, `.two-thirds`)*, inside a single block.
+
+**Extended usage**
+
+- Extend the classes for more semantic ones to your components or use the classes as is. 
+- Create custom width grid items by extending the `grid__item` class and adding widths in a fraction format, with the `span()` function.
+- Add media queries with the `$from` and `$to` variables to change the grid items widths, for your content, on different screen sizes.
+
+*[Demo](http://codepen.io/AntoineBoulanger/pen/EaLNxe)*