document external requirements of FFT convolution

doc/library/tensor/nnet/conv.txt

@@ -32,7 +32,8 @@ TODO: Give examples for how to use these things! They are pretty complicated.
       to perform the work.  You can enable it by setting
       'THEANO_FLAGS=optimizer_including=conv_fft_valid:conv_fft_full'
       in your environement.  This is not enabled by default because it
-      has some restrictions on input and uses more memory.
+      has some restrictions on input and uses more memory.  Also note
+      that it requires CUDA >= 5.0, scikits.cuda >= 0.5.0 and PyCUDA to run.
     - :func:`conv3D <theano.tensor.nnet.Conv3D.conv3D>`. Doesn't work on the GPU.
     - :func:`conv3d2d <theano.tensor.nnet.conv3d2d.conv3d>`
       Another conv3d implementation that uses the conv2d with data reshaping.