Adds tags for discoverability and code snippets

This PR is related to https://github.com/state-spaces/mamba/pull/471 and https://github.com/huggingface/huggingface.js/pull/802.

The goal is to improve discoverability and usability of mamba models by adding tags to the modelcard metadata.

Once PRs are merged, users will see a code snippet in `Use this model` to load the model using `samba_ssm`:

```python
from mamba_ssm import MambaLMHeadModel

model = MambaLMHeadModel.from_pretrained("state-spaces/mamba-130m")
```

This PR is ready for review.

README.md

@@ -1,3 +1,10 @@
 ---
+library_name: mamba-ssm
 license: apache-2.0
+pipeline_tag: text-generation
+tags:
+- arXiv:2312.00752
+- arXiv:2405.21060
+- model_hub_mixin
+- pytorch_model_hub_mixin
 ---