Add CONTRIBUTING.md

[mhpob]

Note

Migrated from meganwinton#8

Developing with rstan and rstantools is not intuitive. rstantools does their best to give direction (links below), but it would be useful to outline how to develop/build the package for those who are interested or come along after us.

https://mc-stan.org/rstantools/articles/developer-guidelines.html
https://mc-stan.org/rstantools/articles/minimal-rstan-package.html#adding-additional-stan-models-to-an-existing-r-package-with-stan-models