Add Terminal.Gui Designer (#3902)

Terminal.Gui Designer is special in the sense it is also a good companion tool, especially for rapid prototyping and for beginners.

README.md

@@ -28,6 +28,8 @@ cd myproj
 dotnet run
 ```
 
+There is also a [visual designer](https://github.com/gui-cs/TerminalGuiDesigner) (uses Terminal.Gui itself).
+
 ## Documentation 
 
 * [Getting Started](https://gui-cs.github.io/Terminal.GuiV2Docs/docs/getting-started.html)