Add attribution

Terminal.Gui/Drawing/Gradient.cs

@@ -1,4 +1,7 @@
-namespace Terminal.Gui;
+// This code is a C# port from python library Terminal Text Effects  https://github.com/ChrisBuilds/terminaltexteffects/
+
+namespace Terminal.Gui;
+
 using System;
 using System.Collections.Generic;
 using System.Linq;