chore: enhance doc

Signed-off-by: lstocchi <[email protected]>

PACKAGING-GUIDE.md

@@ -38,7 +38,7 @@ A model has the following attributes:
 - ```license```: the license under which the model is available
 - ```url```: the URL used to download the model
 - ```memory```: the memory footprint of the model in bytes, as computed by the workflow `.github/workflows/compute-model-sizes.yaml`
-- ```sha256```: the SHA-256 checksum to be used to verify the downloaded model is identical to the original
+- ```sha256```: the SHA-256 checksum to be used to verify the downloaded model is identical to the original. It is optional and it must be HEX encoded  
 
 #### Recipes