Add contributing notes

[fkohrt]

There are some things to consider when contributing to this repo:

• make screenshots in high resolution by temporarily increasing the font scaling factor in your OS and using a large display
• limit the width of images
• annotate code blocks with filename="Terminal" andfilename="Console" to differentiate Bash and R (if there is no file from which the code is an excerpt)
• Put optional content into a collapsed note
• Put important content into warning
• hierarchy of callouts
  • note
  • tip
  • caution
  • warning
  • important
• headings are written in title case
• headings are only H2 and below because the title is already H1
• callouts use context-specific heading levels that are one below the current heading level
• R package names are typeset in monospace
• Prefer R packages that follow the tinyverse philosophy, that is, have a lightweight dependency footprint. All packages need to be downloaded by renv and (worst case) compiled during the tutorial, which better not take too long. Use pak::pkg_deps() to count the total number of package dependencies.

Let's create a CONTRIBUTING.md file that mentions these points.