Added a new Kotlin Playground (#4537)

* Added Blockchain Programming Book

* Added the packt link

* Added a Kotlin Playground

* The Kotlin Playground

free-programming-books.md

@@ -1656,7 +1656,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [JavaScript for Impatient Programmers (ES2020 edition)](https://exploringjs.com/impatient-js) - Dr. Axel Rauschmayer
 * [JavaScript Fundamentals, Plus a Dash Of JQuery - for dinner ladies](http://nicholasjohnson.com/javascript-book/)
 * [JavaScript Garden](http://bonsaiden.github.io/JavaScript-Garden/) (Maintained by Tim Ruffles)
-* [JavaScript Notes for Professionals](https://goalkicker.com/JavaScriptBook/) - Compiled from StackOverflow documentation (PDF) 
+* [JavaScript Notes for Professionals](https://goalkicker.com/JavaScriptBook/) - Compiled from StackOverflow documentation (PDF)
 * [JavaScript Patterns Collection](http://shichuan.github.io/javascript-patterns/) - Shi Chuan
 * [JavaScript Spessore](https://web.archive.org/web/20160325064800/https://leanpub.com/javascript-spessore/read) - Reginald Braithwaite
 * [JavaScript Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/javascript) (PDF, Kindle) (email address *requested*, not required)
@@ -1694,7 +1694,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [AngularJS Notes for Professionals](https://books.goalkicker.com/AngularJSBook) (PDF)
 * [AngularJS Succinctly](https://www.syncfusion.com/resources/techportal/ebooks/angularjs) (PDF, Kindle) (email address *requested*, not required)
 * [AngularJs vs EmberJs](http://angularjs-emberjs-compare.bguiz.com)
-* [Build a Full-Stack Web Application Using Angular & Firebase](https://www.c-sharpcorner.com/ebooks/build-a-full-stack-web-application-using-angular-and-firebase) - Ankit Sharma 
+* [Build a Full-Stack Web Application Using Angular & Firebase](https://www.c-sharpcorner.com/ebooks/build-a-full-stack-web-application-using-angular-and-firebase) - Ankit Sharma
 * [Practical Angular: Build your first web apps with Angular 8](https://leanpub.com/practical-angular) - Ahmed Bouchefra *(Leanpub account or valid email requested)*
 * [Seven-Part Introduction to Angular](http://ngokevin.com/blog/angular-1/)
 * [Unit Testing Best Practices in AngularJS](http://andyshora.com/unit-testing-best-practices-angularjs.html)

free-programming-playgrounds.md

@@ -126,6 +126,7 @@
 ### Kotlin
 
 * [Kotlin](https://play.kotlinlang.org)
+* [Kotlin Playground](https://developer.android.com/training/kotlinplayground)
 
 
 ### Kubernetes