Add MongoDB cheatsheet (#4571)

* Add MongoDB cheatsheet

I couldn't find this covered in an existing PR, linked cheatsheet is my current go-to.

* Add new section to Index

* Appease linter

free-programming-cheatsheets.md

@@ -7,6 +7,7 @@
 * [JavaScript](#javascript)
   * [Vue.js](#vuejs)
 * [Markdown](#markdown)
+* [MongoDB](#mongodb)
 * [Python](#python)
 
 
@@ -46,6 +47,11 @@
 * [Markdown Here](https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet) - Adam Pritchard
 
 
+### MongoDB
+
+* [Quick Cheat Sheet for Mongo DB Shell commands](https://gist.github.com/michaeltreat/d3bdc989b54cff969df86484e091fd0c) - Michael Treat's Quick Cheat Sheet
+
+
 ### Python
 
 * [Comprehensive Python Cheatsheet](https://gto76.github.io/python-cheatsheet) - Jure Šorn (HTML)