Added norm freezing doc

Norm freezing isn't always applied for all version of the model.
Speciying in the hyperparams section that this parameter can be controlled to get the same behaviour as in original implementation.

README.md

@@ -141,6 +141,8 @@ Images are resized to 640x640 pixels and rescaled with `image_mean=[0.485, 0.456
 
 ![image/png](https://cdn-uploads.huggingface.co/production/uploads/6579e0eaa9e58aec614e9d97/E15I9MwZCtwNIms-W8Ra9.png)
 
+Freezing BN is `True` by default for R18 and R34 versions due to backwards compatibility. But in the original implementation the value is `False`.
+Set `freeze_backbone_batch_norms=False` for original model behaviour.
 
 ## Evaluation