update usage

README.md

@@ -4,6 +4,8 @@ language:
 license: "apache-2.0"
 ---
 
+**Please use `ElectraForPreTraining` for `discriminator` and `ElectraForMaskedLM` for `generator` if you are re-training these models.**
+
 ## Chinese ELECTRA
 Google and Stanford University released a new pre-trained model called ELECTRA, which has a much compact model size and relatively competitive performance compared to BERT and its variants.
 For further accelerating the research of the Chinese pre-trained model, the Joint Laboratory of HIT and iFLYTEK Research (HFL) has released the Chinese ELECTRA models based on the official code of ELECTRA.
@@ -40,4 +42,4 @@ If you find our resource or paper is useful, please consider including the follo
     url = "https://www.aclweb.org/anthology/2020.findings-emnlp.58",
     pages = "657--668",
 }
-```
+```