Add app.json contributors to documentation and validator

[mjroeleveld]

{
  ...
  contributors: {
    developers: [{
      email: 'joachim@athom.nl'
    }],
    translators: developers: [{
      email: 'joachim@athom.nl',
      language: 'en' // E.g. (future?)
    }],
  }
  ...
}

[WeeJeWel]

Fixed