Adding recommendation to follow SPDX standard for License field

Hi,

I think it would be nice to recommend usage of [SPDX IDs](https://spdx.org/licenses/) for the license field in the package.json files. That would help reducing ambiguity (e.g. BSD can refer to many different licenses : BSD-2-Clause-FreeBSD, BSD-3-Clause, BSD-4-Clause, etc. ) and improving interoperability.
 
Thanks, 
Camille