i tested the installation with slashs and without slashs - both worksn't

{
  "name": "horizontal homescreen-p",
  "description": "horizontal homescreen beta-version",
  "launch_path": "/index.html",
  "icons": {
      "128": "/img/medienknollen-128.png",
    "512": "/img/medienknollen-512.png"
  },
  "developer": {
    "name": "Gustav Wall",
    "url": "http://sprechrun.de"
  },
  "default_locale": "de"
}