update changes.rst
fix api
'Numeric position' field changed from required to optional.
More than one device can have 'Barcode' field set to None.
Merge pull request #599 from xor-xor/dev2

More than one device can have 'Barcode' field set to None.
Merge pull request #598 from xor-xor/dev1

'Numeric position' field changed from required to optional.
Merge pull request #596 from wmatyskiewicz/api

update changes.rst