READMEs

[Jolanrensen]

Fixes https://github.com/Kotlin/dataframe/issues/844

Please @koperagen and @zaleslaw check the READMEs contents for correctness. I dove into each module and folder and wrote what it contained to my best understanding, but if I made any mistakes, let me know :)

Let's hope this helps us and contributors to no longer get lost in the project :)

Spawns two new issues that can be resolved later:

• https://github.com/Kotlin/dataframe/issues/898
• https://github.com/Kotlin/dataframe/issues/899

[adam-enko]

Converted to a draft, while we discuss further in #3863