Add a Contribution Guide

Many Kotlin libraries like [KotlinDL](https://github.com/Kotlin/kotlindl/blob/master/CONTRIBUTING.md) or [kotlinx.coroutines](https://github.com/Kotlin/kotlinx.coroutines/blob/master/CONTRIBUTING.md) has pretty clear guides on how to contribute to the library

I suggest adding a clear guide and refer to this guide in the future.