Improve model card metadata: pipeline tag and library_name

This PR improves the model card by updating the `pipeline_tag` to `text-to-image` for better discoverability, accurately reflecting the model's text-guided image customization capabilities. It also adds `library_name: transformers` to the metadata, as the model utilizes a `transformers`-based image encoder (`google/siglip-so400m-patch14-384`).

README.md

@@ -1,10 +1,12 @@
 ---
 base_model:
 - black-forest-labs/FLUX.1-Fill-dev
-pipeline_tag: image-to-image
+pipeline_tag: text-to-image
+library_name: transformers
 tags:
 - art
 ---
+
 # Calligrapher: Freestyle Text Image Customization
 
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