Update README.md

Author: aigcliu

README.md

@@ -92,3 +92,4 @@ python train.py --config configs/quick_start/bisenet_optic_disc_512x512_1k.yml
 ## Acknowledgement
 * Thanks [jm12138](https://github.com/jm12138) for contributing U<sup>2</sup>-Net.
 * Thanks [zjhellofss](https://github.com/zjhellofss) (Fu Shenshen) for contributing Attention U-Net, and Dice Loss.
+* Thanks [liuguoyu666](https://github.com/liguoyu666) for contributing U-Net++.