Add library_name to metadata

This PR adds the `library_name: diffusers` tag to the model card metadata to improve discoverability and provide clarity for users.

README.md

@@ -1,11 +1,13 @@
 ---
-license: apache-2.0
-language:
-- en
 base_model:
 - PixArt-alpha/PixArt-XL-2-1024-MS
+language:
+- en
+license: apache-2.0
 pipeline_tag: image-to-image
+library_name: diffusers
 ---
+
 # 🎨 Cobra
 
 **Efficient Line Art COlorization with BRoAder References**