add Cheat Sheet Open Letter

Author: afeld

resources.md

@@ -23,6 +23,7 @@ Make sure to omit any passwords, API keys, or other sensitive information.
 * [Code Crew blog](http://blog.codecrew.co/)
 * [Journeyman](https://plus.google.com/communities/113669203485569960566?cfem=1)
 * [List of Free Programming Books](http://resrc.io/list/10/list-of-free-programming-books/)
+* [Open Letter: Cheat Sheet](http://afeld.me/nerdery/4010407)
 * [Open Letter to a Prospective Programmer](http://afeld.me/nerdery/500322)
 * [Teach Yourself to Code](http://teachyourselftocode.com/) (list of coding resources by topic)