scikit-learn is a production ready library that has some very well commented and easy to read source code.

https://github.com/eriklindernoren/ML-From-Scratch is an easy to understand understand implementation of some the basic ML algorithms built from first principles and aims for readability over performance.

I was asking about the ones which don't have very good documentation and are in need of contributors.

I find that the XGBoost documentation is pretty lacking.