Update README.md

Author: maxbiostat

ProgrammingProjects/README.md

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 - [Efficient implementation of the phylogenetic likelihood and its gradients for use in Stan](https://github.com/maxbiostat/Student_projects/blob/main/ProgrammingProjects/PhyloGradients.md). Languages: C++, Python.
 - [Principled truncation of infinite series in Python](https://github.com/maxbiostat/Student_projects/blob/main/ProgrammingProjects/SumPy.md). Languages: C, Python.
 - [Adaptive truncation for the Conway-Maxwell Poisson in **brms**](https://github.com/maxbiostat/Student_projects/blob/main/ProgrammingProjects/COMP_brms.md). Language: Stan.
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+- [Porting birth-death exact probabilities into  Stan](https://github.com/maxbiostat/Student_projects/blob/main/ProgrammingProjects/MultiBD_Stan.md). Languages: C++, Stan.