Improve model card, add pipeline tag

This PR improves the model card by adding the `pipeline_tag: question-answering` to better categorize the model and linking the Github repository.

README.md

@@ -1,4 +1,8 @@
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 license: apache-2.0
+pipeline_tag: question-answering
 ---
-The repository for the paper "LADDER: Self-Improving LLMs Through Recursive Problem Decomposition". The paper can be found [here](https://arxiv.org/abs/2503.00735).
+
+The repository for the paper "LADDER: Self-Improving LLMs Through Recursive Problem Decomposition". The paper can be found [here](https://arxiv.org/abs/2503.00735).
+
+Code: https://github.com/jzhang-research/ladder