Update CONTRIBUTORS.md

CONTRIBUTORS.md

@@ -9,6 +9,8 @@ Committers are people who have made substantial contribution to the project and
   - Tianqi is a PhD working on large-scale machine learning, he is the creator of the project.
 * [Tong He](https://github.com/hetong007), Simon Fraser University
   - Tong is a master student working on data mining, he is the maintainer of xgboost R package.
+* [Vadim Khotilovich](https://github.com/khotilov)  
+  - Vadim contributes many improvements in R and core packages.
 * [Bing Xu](https://github.com/antinucleon)
   - Bing is the original creator of xgboost python package and currently the maintainer of [XGBoost.jl](https://github.com/antinucleon/XGBoost.jl).
 * [Michael Benesty](https://github.com/pommedeterresautee)
@@ -37,7 +39,6 @@ List of Contributors
   - Zygmunt is the master behind the early stopping feature frequently used by kagglers.
 * [Ajinkya Kale](https://github.com/ajkl)
 * [Boliang Chen](https://github.com/cblsjtu)
-* [Vadim Khotilovich](https://github.com/khotilov)
 * [Yangqing Men](https://github.com/yanqingmen)
   - Yangqing is the creator of xgboost java package.
 * [Engpeng Yao](https://github.com/yepyao)