docs: add kotlin guide to change version (#1185)

This PR adds information on how to override the Kotlin version used by screens and the minimum Kotlin required version.

README.md

@@ -35,6 +35,26 @@ protected void onCreate(Bundle savedInstanceState) {
 
 For people that must handle cases like this, there is [a more detailed discussion of the difficulties in a series of related comments](https://github.com/software-mansion/react-native-screens/issues/17#issuecomment-424704633).
 
+<details>
+<summary>Need to use a custom Kotlin version?</summary>
+<br>
+
+Since `v3.6.0` `react-native-screens` has been rewritten with Kotlin. Kotlin version used in this library defaults to `1.4.10`.
+
+If you need to use a different Kotlin version, set `kotlinVersion` ext property in your project's `android/build.gradle` and the library will use this version accordingly:
+
+```
+buildscript {
+    ext {
+        ...
+        kotlinVersion = "1.4.10"
+    }
+}
+```
+
+**Disclaimer**: `react-native-screens` requires Kotlin `1.3.50` or higher.
+</details>
+
 ### Windows
 
 Installation on Windows should be completely handled with auto-linking when using React Native Windows 0.63+. For earlier versions, you must [manually link](https://microsoft.github.io/react-native-windows/docs/native-modules-using) the native module.