Update Model Card with License

The model card has not been updated with the license.
This seems to be Apache license, though the model card doesn't have it.
The license is in their official repo https://github.com/DachengLi1/LongChat/blob/longeval/LICENSE
It would be great if someone could check and approve this PR.

README.md

@@ -0,0 +1,3 @@
+---
+license: apache-2.0
+---