Mention complex number support in README. (#197)

Author: vincentqb

README.md

@@ -116,7 +116,7 @@ dimension (channel, time)")
 * `min_freq`: the lowest frequency of the lowest band in a spectrogram
 * `max_freq`: the highest frequency of the highest band in a spectrogram
 * `win_length`: the length of the STFT window
-* `window_fn`: for functions that creates windows e.g. torch.hann_window
+* `window_fn`: for functions that creates windows e.g. `torch.hann_window`
 
 Transforms expect the following dimensions.
 
@@ -129,6 +129,8 @@ Transforms expect the following dimensions.
 * `MuLawDecode`: (channel, time) -> (channel, time)
 * `Resample`: (channel, time) -> (channel, time)
 
+Complex numbers are supported via tensors of dimension (..., 2), and torchaudio provides `complex_norm` and `angle` to convert such a tensor into its magnitude and phase.
+
 Contributing Guidelines
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