Add 'sentence-transformers' tag

Hello!

## Pull Request overview
- Add 'sentence-transformers' tag

## Preface
Congratulations on your model release! I'll be diving into the paper in a bit, but I quite like the approach of different loss functions for different dataset formats - well done.

## Details
This tag adds a "Use in Sentence Transformers" button, increases the findability of the model, and makes the model visible when filtering on "Sentence Transformers" in MTEB.

- Tom Aarsen

README.md

@@ -1,6 +1,7 @@
 ---
 tags:
 - mteb
+- sentence-transformers
 model-index:
 - name: piccolo-large-zh-v2
   results: