Added Rextester to R Interactive Playgrounds (#4604)

* Added Kaggle Python course

* Add Python Runestone to interactive tutorials

* added author

* Added Python and Java playground

* add space back

* lower cased name

* fixed capitalization for Java and JavaScript

* Added Rextester to R Interactive Playgrounds

* swapped order

* switched order

free-programming-playgrounds.md

@@ -176,6 +176,7 @@
 ### R
 
 * [R-Fiddle](http://www.r-fiddle.org)
+* [Rextester](https://rextester.com/l/r_online_compiler)
 
 
 ### React