Fold long content on GitHub Flavored Markdown.
When you click the triangle, will toggle the folding content:
So that you can fold contents that only need to see when necessary.
Wrap your long content in details tag and provide a summary:
<details> <summary>The title for folding content</summary> very long content starts here ... very long content ends here </details>
See the real-world example from apple/swift's Pull Request template.
And you can also provide contributor guideline and issue template!