Add `base_model` to let HF make a link between the models

Hello!

## Pull Request overview
* Add `base_model` to let HF make a link between the models

## Details
With this `base_model`, HF can place this model into a Model tree, e.g. like:


- Tom Aarsen

README.md

@@ -21,6 +21,8 @@ widget:
   - One, two, three... Who can guess the next digits?
 pipeline_tag: sentence-similarity
 library_name: sentence-transformers
+base_model:
+- nomic-ai/modernbert-embed-base
 ---
 
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