Update README.md

The GPT-SW3 model is truly impressive! We would love to contribute by updating the README to include information about the base model. This will help address the current gap in the model card.

README.md

@@ -6,6 +6,8 @@ language:
 - 'no'
 - en
 - is
+base_model:
+- AI-Sweden-Models/gpt-sw3-126m
 ---
 # Model description
 [AI Sweden](https://huggingface.co/AI-Sweden-Models/)