docs(contributing): Add playground definition (#6822)

* Added playground definition

Added a small definition about programming/coding playgrounds for making it understandable to beginners

* Define what is a playground

Via / completes:
- What's a programming playground? #6107
- Added playground definition #6817
- Sentence added under playground #6819

Thanks @Suman373 for the initial idea.

Co-authored-by: Suman Roy <95040233+Suman373@users.noreply.github.com>

* reword playground definition as suggested

Co-authored-by: Eric Hellman <eric@hellman.net>

Co-authored-by: Suman Roy <95040233+Suman373@users.noreply.github.com>
Co-authored-by: Eric Hellman <eric@hellman.net>

docs/CONTRIBUTING.md

@@ -18,11 +18,12 @@ By contributing you agree to respect the [Code of Conduct](CODE_OF_CONDUCT.md) o
 2. You don't have to know Git: if you found something of interest which is *not already in this repo*, please open an [Issue](https://github.com/EbookFoundation/free-programming-books/issues) with your links propositions.
     - If you know Git, please Fork the repo and send Pull Requests (PR).
 
-3. We have 5 kinds of lists. Choose the right one:
+3. We have 6 kinds of lists. Choose the right one:
 
     - *Books* : PDF, HTML, ePub, a gitbook.io based site, a Git repo, etc.
     - *Courses* : A course is a learning material which is not a book. [This is a course](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-006-introduction-to-algorithms-fall-2011/).
     - *Interactive Tutorials* : An interactive website which lets the user type code or commands and evaluates the result (by "evaluate" we don't mean "grade"). e.g.: [Try Haskell](http://tryhaskell.org), [Try GitHub](http://try.github.io).
+    - *Playgrounds* : are online and interactive websites, games or desktop software for learning programming. Write, compile (or run), and share code snippets. Playgrounds often allow you to fork and get your hands dirty by playing with code.
     - *Podcasts and Screencasts* : Podcasts and screencasts.
     - *Problem Sets & Competitive Programming* : A website or software which lets you assess your programming skills by solving simple or complex problems, with or without code review, with or without comparing the results with other users.