Add free playgrounds for Crystal

free-programming-playgrounds.md

@@ -2,6 +2,7 @@
 
 * [Angular](#angular)
 * [CSS](#css)
+* [Crystal](#crystal)
 * [FlexBox](#flexbox)
 * [JavaScript](#javascript)
 * [Node.js](#nodejs)
@@ -24,6 +25,11 @@
 * [Dabblet](http://dabblet.com)
 
 
+### Crystal
+
+* [Compile & run code in Crystal](https://play.crystal-lang.org/#/cr)
+
+
 ### FlexBox
 
 * [Echoplex](http://the-echoplex.net/flexyboxes/)