Update README with zenodo reference

Author: mmayers12

README.md

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 # indication_moa_db
 A database of paths that represent the mechanism of action from a drug to a disease in an indication.
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+> [**Database of paths of mechanism of action for selected drug-disease indications**](https://zenodo.org/record/3515487)<br> <Small>Mayers, Michael; Steinecke, Dylan; Su, Andrew I.<small><br>
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.3515487.svg)](https://doi.org/10.5281/zenodo.3515487)
+