Update README

Author: jgraving

README.md

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 behavelet: a wavelet transform for mapping behavior
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+<img src="https://github.com/jgraving/behavelet/blob/master/assets/wavelet.png" max-height:256px>
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 behavelet is a Python implementation of the normalized Morlet wavelet transform for behavioral mapping from [Berman et al. (2014)](https://doi.org/10.1098/rsif.2014.0672).