Mark project as deprecated with alternatives

Added deprecation notice and alternative libraries.

Author: 2A5F

README.md

@@ -6,6 +6,12 @@
 
 Unsafe C# bindings for SDL3
 
+### ***Deprecated***
+
+Alternatives:
+- https://github.com/ppy/SDL3-CS
+- https://github.com/bottlenoselabs/SDL3-cs
+
 ### Packages
 - [![Nuget](https://img.shields.io/nuget/v/Coplt.Sdl3)](https://www.nuget.org/packages/Coplt.Sdl3/)  
   [Coplt.Sdl3](https://www.nuget.org/packages/Coplt.Sdl3/)