Add sentence-transformers tag; will help with findability & MTEB filtering

Hello!

## Pull Request overview
* Add sentence-transformers tag

## Details
This should allow the model to stay visible when filtering for Sentence Transformers compatible models in MTEB.

- Tom Aarsen

README.md

@@ -3,6 +3,7 @@ datasets:
 - allenai/c4
 library_name: transformers
 tags:
+- sentence-transformers
 - gte
 - mteb
 license: apache-2.0