Add Silly.

README.md

 * [Lumen](https://lumen.laravel.com) - A micro-framework by Laravel.
 * [Mezzio](https://getexpressive.org/) - A micro-framework by Laminas.
 * [Radar](https://github.com/radarphp/Radar.Adr) - An Action-Domain-Responder implementation for PHP.
+* [Silly](https://github.com/mnapoli/silly) - A micro-framework for CLI applications.
 * [Slim](https://www.slimframework.com/) - Another simple micro framework.
 
 ### Micro Framework Extras