Adding `summarization` tag to YAML

Adding the `tags: summarization` makes the model more discoverable on the huggingface hub, particularly when people filter for a particular task.

Best,
Dennis

README.md

@@ -3,6 +3,7 @@ language:
 - de
 tags:
 - generated_from_trainer
+- summarization
 metrics:
 - rouge
 model-index: