Update License and Dataset field in the model card

Since the google-research pegasus code is licensed under Apache 2.0 I assume the models are also Apache 2.0. If it's not the case can you provide the proper license since I want to use this model to do some evaluation and my organization wants to know the license terms before I can download and use the model.

I also update the datasets field since it's fine-tuned with the cnn dailymail corpus.

README.md

@@ -2,6 +2,9 @@
 language: en
 tags:
 - summarization
+license: apache-2.0
+datasets:
+- cnn_dailymail
 ---
 
 ### Pegasus Models