Update readme.md

Added DOI

Author: nicpittman

readme.md

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 ## Tropical Pacific Chlorophyll Algorithm (TPCA): Source code and matchups (Pittman et al., 2019) 
 
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3538213.svg)](https://doi.org/10.5281/zenodo.3538213)
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 ### Contact:
 
 Nicholas Pittman, PhD Candidate: Biogeochemistry & Satellite remote sensing. 
 Institute of Marine and Antarctic Studies (IMAS), University of Tasmania, Australia. 
 Australian Research Council Center of Excellence for Climate Extremes (CLEX)
 
 email: nic.pittman@utas.edu.au
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3538213.svg)](https://doi.org/10.5281/zenodo.3538213)
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 ### Contents
 
 - chl_tpca_algorithms.py