[Documentation] Explain how to display math formulas in markdown

[mlange-42]

Possible solution

• Recommend the use of RStudio as Markdown editor if formulas are required.
• For GitHub, see A hack for showing LaTeX formulas in GitHub markdown.md

See also #21

[mlange-42]

Should be done in The Yarner's Book. See #21

[mlange-42]

Done in #111