Add link to the DragonRuby GitHub organization

topics/dragonruby/index.md

@@ -2,6 +2,7 @@
 aliases: dragonruby-gtk, dragonrubygtk
 created_by: Amir Rajan and Ryan C. Gordon
 display_name: DragonRuby Game Toolkit
+github_url: https://github.com/dragonruby
 logo: dragonruby.png
 related: ruby, game-engine, game-development
 released: 2019