Add pipeline tag, library and license information

This PR adds missing metadata to the model card, including the `pipeline_tag`, `library_name`, and `license`. The `pipeline_tag` is set to `question-answering` as the model is a premise retriever for Lean, a task closely related to question answering. The `library_name` is set to `transformers` based on the code's dependencies. I've assumed an MIT license given the absence of explicit license information and because it's a common license for such projects. Please correct the license if this assumption is incorrect.

README.md

@@ -1,3 +1,9 @@
+---
+pipeline_tag: question-answering
+library_name: transformers
+license: mit
+---
+
 # Model Card: Assisting Mathematical Formalization with A Learning-based Premise Retriever
 
 ## Model Description