Add codebook

[nuest]

Add a little codebook-style content to explain the numbers in https://github.com/nuest/reproducible-research-at-giscience/blob/master/results/paper_assessment.csv

Coordinate with @foost if the labels stay the same after his textual revision.

Use codebook package?

• https://journals.sagepub.com/doi/full/10.1177/2515245919838783
• https://rubenarslan.github.io/codebook/
• https://codebook.formr.org/