Add guidelines for repo structure

[leewujung]

I posted this some time ago in slack, but probably better to have these in the group docs so they are easy to find.

Some resources that stemmed back from a 2009 article that is worth referencing here:

• medium post: https://medium.com/outlier-bio-blog/a-quick-guide-to-organizing-data-science-projects-updated-for-2016-4cbb1e6dac71
• original paper: https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1000424
• yet another cookiecutter from the blog author (I don’t think we’ll just use it): https://github.com/outlierbio/ob-project-template