Update free-programming-playgrounds.md (#4677)

* Update free-programming-playgrounds.md

* Update free-programming-playgrounds.md

free-programming-playgrounds.md

@@ -194,6 +194,7 @@
 ### Ruby
 
 * [Codepad](http://codepad.org)
+* [Sololearn](https://code.sololearn.com/#rb)
 * [TryRuby](https://try.ruby-lang.org)