add dpo link (huggingface#1502)

README.md

@@ -215,7 +215,7 @@ make dev
 The PPO implementation largely follows the structure introduced in the paper **"Fine-Tuning Language Models from Human Preferences"** by D. Ziegler et al. \[[paper](https://arxiv.org/pdf/1909.08593.pdf), [code](https://github.com/openai/lm-human-preferences)].
 
 ### Direct Preference Optimization
-DPO is based on the original implementation of **"Direct Preference Optimization: Your Language Model is Secretly a Reward Model"** by E. Mitchell et al. \[[paper](), [code](https://github.com/eric-mitchell/direct-preference-optimization)]
+DPO is based on the original implementation of **"Direct Preference Optimization: Your Language Model is Secretly a Reward Model"** by E. Mitchell et al. \[[paper](https://arxiv.org/pdf/2305.18290.pdf), [code](https://github.com/eric-mitchell/direct-preference-optimization)]
 
 
 ## Citation