Add license

This PR adds the correct license, which is a custom one, according to the information in the model card.

README.md

@@ -11,7 +11,9 @@ language:
 base_model:
 - Efficient-Large-Model/Sana_1600M_2Kpx_BF16_diffusers
 pipeline_tag: text-to-image
+license: other
 ---
+
 <p align="center" style="border-radius: 10px">
   <img src="https://raw.githubusercontent.com/NVlabs/Sana/refs/heads/main/asset/logo.png" width="35%" alt="logo"/>
 </p>