add style / form advice to contributing guidelines

[egpbos]

Fixes #150.

Changes in this PR

Checklist

SIGNIFICANT changes / additions, e.g. new chapters

• I checked whether the contribution fits in The Turing Way before considering contributing to this Guide.
• I discussed my contribution in an issue and took into account feedback.

ALL contributions

• I previewed my changes locally using e.g. python3 -m http.server 4000 and confirmed they work correctly.
• I checked for broken links, e.g. using the link checker GitHub Action workflow, or locally by using docker run --init -it -v pwd:/docs lycheeverse/lychee /docs --config=docs/lychee.toml, at least for the files I changed.
• My name was added to the CITATION.cff file.

[bouweandela]

Thanks!