Added a Kotlin section in CONTRIBUTING.md

Core team does not want to convert to Kotlin yet and sees Java as the easier to learn and more well adopted language.

This stance might of course change in the future. For example it could be reasonable to do a complete transition to Kotlin once it is decides that the minSdk is raised to 21 or higher, as we then could use Jetpack particularly Lifecycle and Compose.

.github/CONTRIBUTING.md

@@ -39,6 +39,11 @@ You'll see *exactly* what is sent, be able to add **your comments**, and then se
 * Create PRs that cover only **one specific issue/solution/bug**. Do not create PRs that are huge monoliths and could have been split into multiple independent contributions.
 * NewPipe uses [NewPipeExtractor](https://github.com/TeamNewPipe/NewPipeExtractor) to fetch data from services. If you need to change something there, you must test your changes in NewPipe. Telling NewPipe to use your extractor version can be accomplished by editing the `app/build.gradle` file: the comments under the "NewPipe libraries" section of `dependencies` will help you out.
 
+### Kotlin in NewPipe
+* NewPipeExtractor shall remain Java only
+* NewPipe will remain mostly Java for time being
+* Contributions containing a simple conversion from Java to Kotlin should be avoided. Conversions to Kotlin should only be done if Kotlin actually brings improvements like bug fixes or better performance which are not, or only with much more effort, implementable in Java. The core team sees Java as an easier to learn and generally well adopted programming language.
+
 ### Creating a Pull Request (PR)
 
 * Make changes on a **separate branch** with a meaningful name, not on the _master_ branch or the _dev_ branch. This is commonly known as *feature branch workflow*. You may then send your changes as a pull request (PR) on GitHub.