Add pipeline tag, library name and link to code

This PR improves the model card by:

- linking it to the relevant paper;
- ensuring it can be found at https://huggingface.co/models?pipeline_tag=text-generation
- adding the Transformers library, ensuring the 'How to use' button appears on the top right
- adding a link to the code repository

README.md

@@ -1,3 +1,9 @@
----
-license: mit
----
+---
+license: mit
+library_name: transformers
+pipeline_tag: text-generation
+---
+
+This repository contains the LongSpec model described in [LongSpec: Long-Context Lossless Speculative Decoding with Efficient Drafting and Verification](https://huggingface.co/papers/2502.17421).
+
+For training and usage, see the code at https://github.com/sail-sg/LongSpec.