add note about CUFFT

README.md

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-This package provides a Julia implementation of the Fast Hartley Transform (FHT), using [`FFTW.jl`](https://github.com/JuliaMath/FFTW.jl) as its kernel for the fast Fourier Transform. The implementation is built up on [`AbstractFastHartleyTransforms.jl`](https://github.com/EHTJulia/AbstractFastHartleyTransforms.jl), providing generic interfaces including `fht(x)`, `fht!(x)`, `ifht(x)`, `ifht!(x)`, `plan_fht(x)`, and `plan_ifht(x)`.
+This package provides a Julia implementation of the Fast Hartley Transform (FHT). It supports both CPUs and NVIDIA CUDA GPUs by using following FFT (fast Fourier transform) kernels:
+
+- CPU implementation uses [`FFTW.jl`](https://github.com/JuliaMath/FFTW.jl).
+- NVIDIA GPU implementation uses [`CUDA.jl`](https://github.com/JuliaGPU/CUDA.jl)'s `CUDA.CUFFT` module. 
+
+
+The implementation is built up on [`AbstractFastHartleyTransforms.jl`](https://github.com/EHTJulia/AbstractFastHartleyTransforms.jl), providing generic interfaces including `fht(x)`, `fht!(x)`, `ifht(x)`, `ifht!(x)`, `plan_fht(x)`, and `plan_ifht(x)`.