[WIP] Start grid doc

docs/grid.md

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+# Grid system
+
+* [Architectures](#architectures)
+  * [Styles](#styles)
+  * [Build tasks](#build-tasks)
+    * [Config file](#config-file)
+* [Usage](#usage)
+  * [Example](#example-1)
+* [Limitations](#limitations)
+
+## Architectures
+
+The Grid System is meant to be simple and easy to use. To goal is to obtain way to build light, flexible and reusable way to build layouts.
+
+### Styles
+
+Some styles are needed to make it work properly :
+
+* [`o-grid.scss`](../assets/styles/objects/_grid.scss): Objecy file where the default grid styles are set. Column numbers, modifiers, options.
+* [`u-grid-columns.scss`](../assets/styles/utilities/_grid-column.scss): Utility file where it generates every possible column styles based on an array of media queries and column numbers.
+
+### Build tasks
+
+The generation of columns adds a lot of styles in the css output file. To solve that, a build task is set up to purge unused css generated by [`u-grid-columns.scss`](../assets/styles/utilities/_grid-column.scss) using [PurgeCSS].
+
+#### Config file
+
+Depending on your project, you will need to specify all the files that will include the Grid System classes. These files will be parsed by [PurgeCSS] and then a "cleaned" css file will be generated. The styles should also compile everytime the following listed files are changed.
+
+```json
+// Charcoal project example
+"purgeCSS": {
+    "content": [
+        "./www/**/*.html", 
+        "./views/app/template/**/*.mustache",
+        "./src/App/Template/*.php",
+        "./assets/scripts/**/*" // use case: el.classList.add('u-gc-1/2')
+    ]
+}
+```
+
+## Usage
+
+Test
+
+
+[purgecss]:      https://purgecss.com/