add tailwind-play to free-programming-playgrounds (EbookFoundation#7207)

* add tailwind-play to free-programming-playgrounds

* fix linting, remove trailing slash

Author: ddiebel

more/free-programming-playgrounds.md

@@ -144,6 +144,7 @@
 * [Dabblet](https://dabblet.com)
 * [Flexy Boxes](https://the-echoplex.net/flexyboxes/)
 * [SoloLearn](https://code.sololearn.com/web#css)
+* [Tailwind Play](https://play.tailwindcss.com)
 
 
 ### Ionic