[ref model] Update README for save_npy and load_npy.

- Added documentation on load_npy and save_npy behavior.
- Included usage examples showing support for both string and DType
 inputs.

Signed-off-by: Eirene Vlassi Pandi <eirene.vlassipandi@arm.com>
Change-Id: I84ec2fd385109857633e8da3eaddea03acab9d16