Move English documentation to folder en_US

doc/en_US/coding-standards.markdown

@@ -0,0 +1,24 @@
+Coding standards
+================
+
+PHP code
+--------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`
+- Encoding: UTF-8
+- Use only the opening tags `<?php` or `<?=` for templates, but **never** use `<?`
+- Always write PHPdoc comments for methods and class properties
+- Coding style: [PSR-1](http://www.php-fig.org/psr/psr-1/) and [PSR-2](http://www.php-fig.org/psr/psr-2/)
+
+JavaScript code
+---------------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`
+
+CSS code
+--------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`