README: add PyPI badge

Author: John Garrett

README.md

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 Chirp z-Transform (CZT)
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+[![PyPI version](https://badge.fury.io/py/czt.svg)](https://badge.fury.io/py/czt)
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 From [Wikipedia](https://en.wikipedia.org/wiki/Chirp_Z-transform):
 
 > The chirp Z-transform (CZT) is a generalization of the discrete Fourier transform (DFT). While the DFT samples the Z plane at uniformly-spaced points along the unit circle, the chirp Z-transform samples along spiral arcs in the Z-plane, corresponding to straight lines in the S plane. The DFT, real DFT, and zoom DFT can be calculated as special cases of the CZT.