allow URLs to be used in package.json for license and readme fields

In package.json, I would like to reference URLs for the license and readme fields:

  "author": "Olegzandr V.D.",
  "readme": "https://github.com/ORESoftware/r2g/blob/master/readme.md",
  "license": "https://github.com/ORESoftware/r2g/blob/master/license.md",

but currently I am getting:

npm WARN r2g@0.1.201-rc.0 license should be a valid SPDX license expression