Add license metadata

Add model-card license metadata matching the primary upstream model, [sd2-community/stable-diffusion-2-1-unclip](https://huggingface.co/sd2-community/stable-diffusion-2-1-unclip).

This changes only the README YAML frontmatter.

README.md

@@ -1,4 +1,5 @@
 ---
+license: openrail++
 tags:
 - diffusion-single-file
 - comfyui