Added markdown cheatsheet (#5971)

more/free-programming-cheatsheets.md

@@ -133,6 +133,7 @@
 
 ### Markdown
 
+* [Markdown Cheat Sheet](https://www.markdownguide.org/cheat-sheet/) - Markdown Guide (HTML)
 * [Markdown Here](https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet) - Adam Pritchard