update README

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 ## FFT on the GPU
 
-This is an optimized GPU-based 2D FFT for VRChat. It is only suitable for use
-in worlds.
+This is an optimized GPU-based 2D FFT for VRChat. Since it uses Custom
+RenderTextures (CRTs), it is only suitable for use in worlds.
+
+Please take note that the FFT is generated in bit-reversed order. You'll have
+to bit-reverse your indices when accessing it. See `apply_2d_bit_reversal` in
+`gpu_fft.cc` for an example of how to do that.
 
 ### Quick start
 
+Import the unitypackage and open the scene.
+
 Run CPU simulator:
 
 ```bash