Adding a tiny README file for each module

[Jolanrensen]

I keep having to dig into each module like :tests, :generator, and :symbol-processor to figure out what they do again. If even I have this issue, as a full-time DataFrame developer, I can only imagine what it must be like for other contributors...

Let's add a small README file into each module in the project with a small description what it's for and how it's used.

For inspiration, this is what Dokka has: https://github.com/Kotlin/dokka/tree/master/dokka-subprojects/analysis-kotlin-api