Update model metadata to set pipeline tag to the new `text-ranking` and library name to `sentence-transformers`

Hello!

## Pull Request overview
* Update metadata to set pipeline tag to the new `text-ranking`
* Update metadata to set library name to `sentence-transformers`

## Changes
This is an automated pull request to update the metadata of the model card. We recently introduced the [`text-ranking`](https://huggingface.co/models?pipeline_tag=text-ranking) pipeline tag for models that are used for ranking tasks, and we have a suspicion that this model is one of them. I also updated added metadata to specify that this model can be loaded with the `sentence-transformers` library, as it should be possible to load any model compatible with `transformers` `AutoModelForSequenceClassification`.

Feel free to verify that it works with the following:

```bash
pip install sentence-transformers
```

```python
from sentence_transformers import CrossEncoder

model = CrossEncoder("pjbhaumik/crossencoder-ms-marco-MiniLM-L-12-v2")
scores = model.predict([
("How many people live in Berlin?", "Berlin had a population of 3,520,031 registered inhabitants in an area of 891.82 square kilometers."),
("How many people live in Berlin?", "Berlin is well known for its museums."),
])
print(scores)
```

Feel free to respond if you have questions or concerns.

- Tom Aarsen

README.md

@@ -3,6 +3,8 @@ license: apache-2.0
 base_model: cross-encoder/ms-marco-MiniLM-L-12-v2
 tags:
 - generated_from_trainer
+library_name: sentence-transformers
+pipeline_tag: text-ranking
 model-index:
 - name: crossencoder-ms-marco-MiniLM-L-12-v2
   results: []