Add license information to model cards (#3864)

Close #3357

model_cards/google/electra-base-discriminator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators

model_cards/google/electra-base-generator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators

model_cards/google/electra-large-discriminator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators

model_cards/google/electra-large-generator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators

model_cards/google/electra-small-discriminator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators

model_cards/google/electra-small-generator/README.md

@@ -1,6 +1,8 @@
 ---
 language: english
 thumbnail: https://huggingface.co/front/thumbnails/google.png
+
+license: apache-2.0
 ---
 
 ## ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators