Add base_model metadata (microsoft/Lens)

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:
- microsoft/Lens
```

Setting this will surface the model in microsoft/Lens'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:
+  - microsoft/Lens
 license: mit
 language:
 - en