Define what is a playground

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

Thanks @Suman373 for the initial idea.

Co-authored-by: Suman Roy <[email protected]>

Author: davorpa

Diff for: README.md

@@ -182,8 +182,6 @@ Free Podcasts and Screencasts:
 
 ### Programming Playgrounds
 
-Whether you're a beginner programmer or an expert developer, code/programming playgrounds are useful when sharing and learning with others. A programming playground is an online service where you can write, compile(or run), and share code with others. They also allows you to fork and get your hands dirty by playing around with other's code.
-
 + [Chinese / 中文](more/free-programming-playgrounds-zh.md)
 + [English](more/free-programming-playgrounds.md)
 

Diff for: 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 useful to learn playing. Therefore, you can write, compile (or run), and share this code snippets with others. They also usually allows you to fork and get your hands dirty by playing around with other's 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.