Added flexbox playground and link to playgrounds from readme

Author: tinabme

Diff for: README.md

@@ -70,3 +70,7 @@ Free Podcasts and Screencasts:
 + English: [github](/free-podcasts-screencasts-en.md)
 + Portuguese (Brazil): [github](/free-podcasts-screencasts-pt_BR.md)
 + Russian: [github](/free-podcasts-screencasts-ru.md)
+
+
+### Programming Playgrounds
++ [Free Programming Playgrounds](/free-programming-playgrounds.md)

Diff for: free-programming-playgrounds.md

@@ -2,6 +2,7 @@
 
 * [Angular](#angular)
 * [CSS](#css)
+* [FlexBox](#flexbox)
 * [JavaScript](#javascript)
 * [Node.js](#nodejs)
 * [PHP](#php)
@@ -15,6 +16,11 @@
 * [Plunker](http://plnkr.co)
 
 
+### FlexBox
+
+* [Echoplex](http://the-echoplex.net/flexyboxes/)
+
+
 ### CSS
 
 * [CodePen](http://codepen.io)