Add some doc for contributors

docs/coding-standards.markdown

@@ -0,0 +1,23 @@
+Coding standards
+================
+
+PHP code
+--------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`
+- Encoding: UTF-8
+- Always write PHPdoc comments for methods and class properties
+- Coding style: Try to follow [PSR-1](http://www.php-fig.org/psr/psr-1/) and [PSR-2](http://www.php-fig.org/psr/psr-2/) otherwise follow the actual style.
+
+Javascript code
+---------------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`
+
+CSS code
+--------
+
+- Indentation: 4 spaces
+- Line return: Unix => `\n`