Add pipeline tag to model card

This PR updates the model card by adding the `pipeline_tag: image-feature-extraction` to its metadata. This will make the model discoverable on the Hugging Face Hub under this pipeline and clarify its primary function of extracting features from images.

README.md

@@ -1,5 +1,6 @@
 ---
 license: mit
+pipeline_tag: image-feature-extraction
 ---
 
 **The official model release for SSL4Eco.**