[COMMUNITY] @sgrechanik-h -> Reviewer (#2732)

CONTRIBUTORS.md

@@ -33,6 +33,7 @@ We do encourage everyone to work anything they are interested in.
 - [Tianqi Chen](https://github.com/tqchen): @tqchen
 - [Liangfu Chen](https://github.com/liangfu): @liangfu
 - [Zhi Chen](https://github.com/zhiics): @zhiics
+- [Sergei Grechanik](https://github.com/sgrechanik-h): @sgrechanik-h
 - [Nick Hynes](https://github.com/nhynes): @nhynes
 - [Yuwei Hu](https://github.com/Huyuwei): @Huyuwei
 - [Yizhi Liu](https://github.com/yzhliu) : @yzhliu