[Doc] update contributors (#409)

CONTRIBUTORS.md

@@ -2,15 +2,22 @@
 
 Contribution is always welcomed. A good starting place is the roadmap issue, where
 you can find our current milestones. All contributions must go through pull requests
-and be reviewed by the committers. See our [contribution guide](https://docs.dgl.ai/contribute.html) for more details.
+and be reviewed by the committers. See our [contribution
+guide](https://docs.dgl.ai/contribute.html) for more details.
 
-Once your contribution is accepted and merged, congratulations, you are now a contributor to the DGL project.
-We will put your name in the list below and also on our [website](https://www.dgl.ai/ack).
+Once your contribution is accepted and merged, congratulations, you are now a
+contributor to the DGL project.  We will put your name in the list below.
 
 Contributors
 ------------
-[Yizhi Liu](https://github.com/yzhliu)
-[Yifei Ma](https://github.com/yifeim)
-Hao Jin
-[Sheng Zha](https://github.com/szha)
-[Brett Koonce](https://github.com/brettkoonce)
+* [Sheng Zha](https://github.com/szha): tree-lstm in MXNet
+* [Yifei Ma](https://github.com/yifeim): GCN in MXNet
+* [Yizhi Liu](https://github.com/yzhliu): RGCN in MXNet
+* [@hbsun2113](https://github.com/hbsun2113): GraphSAGE in Pytorch
+* [Tianyi Zhang](https://github.com/Tiiiger): SGC in Pytorch
+
+Other improvement
+* [Brett Koonce](https://github.com/brettkoonce)
+* [@giuseppefutia](https://github.com/giuseppefutia)
+* [@mori97](https://github.com/mori97)
+* Hao Jin