Add xgboost package in Machine Learning section

xgboost is a package implementing Gradient Boosting Tree model, which is first popular among the community of Kaggle, a machine learning competition website. It is well known for the fast speed and outstanding performance. There are numerous competitors won competitions or achieved high rankings with it. 

References:
[Winning Solution](http://www.kaggle.com/c/tradeshift-text-classification/forums/t/10901/solution-sharing)
[Popularity proof](https://www.kaggle.com/c/higgs-boson/forums/t/8184/public-starting-guide-to-get-above-3-60-ams-score)