Define citation template for REMARKs

[llorracc]

In the portfolio choice blog post, I needed to refer to the CGMPortfolio REMARK, and realized that we do not have a recommended or standardized way that we have identified for people to refer to our material.

I propose that we should add, at the bottom of the "materials" page for each item, a copy-and-pasteable bibtex entry that people could use to make reference to our materials trivial. It should include a link to the material itself, a standardized citekey, and the minimal requirements to compile successfully in a standard LaTeX/bibtex setup.

Something like this:

@techreport{CGMPortfolioREMARK, 
title={Replication of `Consumption and Portfolio Choice over the Life Cycle' by Cocco, Gomes, and Maenhout (2005)}, 
url={https://github.com/econ-ark/REMARK/tree/master/REMARKs/CGMPortfolio}, 
journal={Econ-ARK/REMARK}, 
publisher={Econ-ARK},
author={Velazques-Giraldo, Mateo and Zahn, Matthew}, 
year={2020}, 
month={Mar},
doi={doi}
}