Multiple authoritive GUI urls (#602)

* Multiple authoritive GUI urls

* urls that do not comply with the shard rules

README.md

@@ -278,6 +278,7 @@ described in [RFC 8032]
  * [crystal-imgui](https://github.com/oprypin/crystal-imgui) - Bindings to [Dear ImGui](https://github.com/ocornut/imgui), an immediate-mode graphical UI library
  * [GTK4.cr](https://github.com/hugopl/gtk4.cr) - Bindings for [GTK4](https://docs.gtk.org/gtk4/overview.html) with Crystalized API
  * [Iu](https://github.com/grkek/iu) - UI framework based on the [Fusion/libui.cr](https://github.com/Fusion/libui.cr) library, with custom elements and modified bindings from [hedron-crystal/hedron](https://github.com/hedron-crystal/hedron)
+ * [Ultimate GTK4 Crystal Guide](https://ultimate-gtk4-crystal-guide.geopjr.dev/) - Learn how to create premium GTK4 apps in Crystal
 
 ## HTML Builders
  * [form_builder.cr](https://github.com/westonganger/form_builder.cr) - Dead simple HTML form builder for Crystal with built-in support for many popular UI libraries such as Bootstrap