Contributing: add directive about compliance with WPCS

Author: jrfnl

.github/CONTRIBUTING.md

@@ -22,6 +22,7 @@ Found a bug you can fix? Fantastic! Patches are always welcome. Here's a few tip
 * Include the purpose of your PR. Be explicit about the issue your PR solves.
 * Reference any existing issues that relate to your PR. This allows everyone to easily see all related discussions.
 * When submitting a change that affects CSS, please make sure that both SCSS sources and output CSS have been updated equally.
+* `_s` complies with the [WordPress Coding Standards](https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/) and any PR should comply as well.
 
 By contributing code to `_s`, you grant its use under the [GNU General Public License v2 (or later)](LICENSE).