Add CUDA toolkit

Compile-in-Windows.md

@@ -9,6 +9,8 @@
   * Install NSIS (https://nsis.sourceforge.io/Download)
   * Make sure git, cmake, nsis and ninja are in your PATH environment variable.
 
+  * Optionally install CUDA Toolkit 12.6 (https://developer.nvidia.com/cuda-downloads?target_os=Windows&target_arch=x86_64&target_version=11&target_type=exe_local)
+
 ## Compile and Build (cmd line)
 
 Run Visual Studio shell: `Start > Visual Studio 2022 > x64 Native Tools Command Prompt for VS 2022` Then: