add reference to GPU acceleration paper (#756)

docs/GPU-Performance.md

@@ -174,4 +174,10 @@ significant when the dataset is small.
 | GPU Memory Usage (MB) | 611   | 901     |  1067  |   413   |  405  |  291     |
 
 
+Further Reading
+--------------------------
+
+You can find more details about the GPU algorithm and benchmarks in the following article:
+
+Huan Zhang, Si Si and Cho-Jui Hsieh. [GPU Acceleration for Large-scale Tree Boosting](https://arxiv.org/abs/1706.08359). arXiv:1706.08359, 2017.
 

docs/GPU-Tutorial.md

@@ -171,3 +171,10 @@ Further Reading
 
 [GPU Windows Tutorial](./GPU-Windows.md)
 
+Reference
+---------------
+
+Please kindly cite the following article in your publications if you find the GPU acceleration useful:
+
+Huan Zhang, Si Si and Cho-Jui Hsieh. [GPU Acceleration for Large-scale Tree Boosting](https://arxiv.org/abs/1706.08359). arXiv:1706.08359, 2017.
+