message: Add Zenodo DOI for citations.
diff: * a full first- and higher-order automatic differentiation library based on C++ template overloads, and
  * a supporting fully-templated matrix, linear algebra, and probability special function library.
  +[![DOI](https://zenodo.org/badge/16967338.svg)](https://zenodo.org/badge/latestdoi/16967338)
  +
  ### Home Page
  Stan's home page, with links to everything you'll need to use Stan is:
  