[Doc] Update list of winning solutions in data science competitions using XGBoost

The demo directory shows a list of winning solutions for data science competitions that used XGBoost: https://github.com/dmlc/xgboost/blob/master/demo/README.md#machine-learning-challenge-winning-solutions

The list hasn't been updated since October 2017. It would be great if we can update this list.

How to update the list.

https://www.kaggle.com/competitions is a great place to start. For each completed competition, look for the first, second, and third-place solutions. If they used XGBoost (even as part of ensemble), create a pull request to add them to https://github.com/dmlc/xgboost/blob/master/demo/README.md.

There are other data science competitions as well. For example, XGBoost was used in the first winning solution of ACM RecSys 2020 Challenge.