kotlin cheatsheet (EbookFoundation#6417)

Author: PyromancerBoom

Diff for: more/free-programming-cheatsheets.md

@@ -18,6 +18,7 @@
   * [Nuxt.js](#nuxtjs)
   * [React.js](#reactjs)
   * [Vue.js](#vuejs)
+* [Kotlin](#Kotlin)
 * [Kubernetes](#kubernetes)
 * [Language Translations](#language-translations)
 * [Markdown](#markdown)
@@ -152,6 +153,11 @@
 * [Vue.js 2.3 Complete API Cheat Sheet](https://marozed.com/vue-cheatsheet) - Marcos Neves, Marozed (HTML)
 
 
+### Kotlin
+
+* [Kotlin Cheatsheet and Quick Reference](https://koenig-media.raywenderlich.com/uploads/2019/11/RW-Kotlin-Cheatsheet-1.1.pdf) - Ray Wenderlich (PDF)
+
+
 ### Kubernetes
 
 * [Handy Cheat Sheet for Kubernetes Beginners](https://kubernetes.io/docs/reference/kubectl/cheatsheet/) - Kubernetes Documentation: kubectl Cheat Sheet