Add Ruby Cheat Sheet (#4851)

free-programming-cheatsheets.md

@@ -12,6 +12,7 @@
 * [Markdown](#markdown)
 * [MongoDB](#mongodb)
 * [Python](#python)
+* [Ruby](#ruby)
 * [SQL](#sql)
 
 
@@ -85,6 +86,11 @@
 * [Python WorldWide Cheatsheet](https://cheatography.com/davechild/cheat-sheets/python/) - Cheatography (Python)
 
 
+### Ruby
+
+* [Ruby Cheat Sheet](https://www.codeconquest.com/wp-content/uploads/Ruby-Cheat-Sheet-by-CodeConquestDOTcom.pdf) - CodeConquest.com (PDF)
+
+
 ### SQL
 
 * [MySQL Cheatsheet](https://s3-us-west-2.amazonaws.com/dbshostedfiles/dbs/sql_cheat_sheet_mysql.pdf) - Database Star (PDF)