Add base_model metadata (nreimers/MiniLM-L6-H384-uncased)

This repo currently has no `base_model` set in its card metadata, so it doesn't appear in any model's tree:

```yaml
base_model:
- nreimers/MiniLM-L6-H384-uncased
```

Setting this will surface the model in nreimers/MiniLM-L6-H384-uncased's **Model Tree** (under its quantizations/finetunes) and improve discoverability for people browsing from the base model.

Feel free to merge directly (or close).

README.md

@@ -1,4 +1,6 @@
 ---
+base_model:
+  - nreimers/MiniLM-L6-H384-uncased
 language: en
 license: apache-2.0
 library_name: sentence-transformers