Move English documentation to folder en_US

doc/en_US/assets.markdown

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+How to build assets (Javascript and CSS files)
+==============================================
+
+Stylesheet and Javascript files are merged together and minified.
+
+- Original CSS files are stored in the folder `assets/css/src/*.css`
+- Original Javascript code is stored in the folder `assets/js/src/*.js`
+- `assets/*/vendor.min.*` are external dependencies merged and minified
+- `assets/*/app.min.*` are application source code merged and minified
+
+Requirements
+------------
+
+- [NodeJS](https://nodejs.org/) with `npm`
+
+Building Javascript and CSS files
+---------------------------------
+
+Kanboard use [Gulp](http://gulpjs.com/) to build the assets and [Bower](http://bower.io/) to manage dependencies.
+These tools are installed as NodeJS dependencies into the project.
+
+### Run everything
+
+```bash
+make static
+```
+
+### Build `vendor.min.js` and `vendor.min.css`
+
+```bash
+gulp vendor
+```
+
+### Build `app.min.js`
+
+```bash
+gulp js
+```
+
+### Build `app.min.css`
+
+```bash
+gulp css
+```
+
+Notes
+-----
+
+Building assets is not possible from the Kanboard's archive, you have to clone the repository.