Add Cecil as static site generator (cc @ArnaudLigny)

README.md

 * [Couscous](http://couscous.io) - Couscous turns Markdown documentation into beautiful websites. It's GitHub Pages on steroids.
 * [Jigsaw](https://jigsaw.tighten.com/) - Simple static sites with Laravel's Blade.
 * [Sculpin](https://sculpin.io) - A tool that converts Markdown and Twig into static HTML.
+* [Cecil](https://cecil.app/) - A simple and powerful content-driven static site generator.
 
 ### HTTP
 *Libraries for working with HTTP.*