Update README.md

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README.md

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 Minimalist Matlab implementation of a random process generation in one point
 
 [![View One-point random process generation on File Exchange](https://www.mathworks.com/matlabcentral/images/matlab-file-exchange.svg)](https://se.mathworks.com/matlabcentral/fileexchange/76854-one-point-random-process-generation)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3890406.svg)](https://doi.org/10.5281/zenodo.3890406)
 
 ## Summary
 A stationary Gaussian random process is generated using the spectral method. This means that the function requires only two inputs: the target power spectral density (PSD) and the associated frequency vector.