Added Rextester to R Interactive Playgrounds

[kevinlwebb]

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What does this PR do?

Add Resource(s)

For resources

Description

Rextester provides playgrounds for multiple coding languages including R

Why is this valuable (or not)?

1. Free
2. Interactive

How do we know it's really free?

You can freely access the website.

For book lists, is it a book? For course lists, is it a course? etc.

Playground

Checklist:

• Not a duplicate
• Included author(s) if appropriate
• Lists are in alphabetical order
• Needed indications added (PDF, access notes, under construction)

[kevinlwebb]

@eshellman I have tried putting my edit before and after R-Fiddle, but neither work.

[SethFalco]

@eshellman I have tried putting my edit before and after R-Fiddle, but neither work.

Heads up, you can check Travis CI manually if you want to see exactly what's wrong! There's no need to do trial-and-error commits!
- https://travis-ci.com/github/EbookFoundation/free-programming-books/builds/191563619

Currently, it displays:

home/travis/build/EbookFoundation/free-programming-books/free-programming-playgrounds.md
  178:1-179:38  warning  Alphabetical ordering: swap l.179 and l.178  alphabetize-lists  remark-lint

So, this is the correct order:

* [R-Fiddle](http://www.r-fiddle.org)
* [Rextester](https://rextester.com/l/r_online_compiler)

Can you please fix the order of your list!

Meanwhile, don't worry about the build failures, it's not your fault. It seems some commits that had failures were merged is all!