Update free-programming-books.md (#3199)

* Update free-programming-books.md

* Update free-programming-books.md

Sorry for the error, fixed the spaces so that the linting will now work.

free-programming-books.md

@@ -97,6 +97,7 @@
   * [Redux](#redux)
 * [Jenkins](#jenkins)
 * [Julia](#julia)
+* [Kotlin](#Kotlin)
 * [Language Agnostic](#language-agnostic)
   * [Algorithms & Data Structures](#algorithms--data-structures)
   * [Cellular Automata](#cellular-automata)
@@ -1682,6 +1683,12 @@ Kerridge (PDF) (email address *requested*, not required)
 * [The Julia Express](http://bogumilkaminski.pl/files/julia_express.pdf) - Bogumił Kamiński (PDF)
 
 
+### Kotlin
+
+* [Kotlin Notes for Professionals book](https://goalkicker.com/KotlinBook/) - Compiled from StackOverflow documentation
+* [Kotlin Official Documentation](https://kotlinlang.org/docs/reference/)
+
+
 ### LaTeX / TeX
 
 #### LaTeX