Examine source code of ReadPosition

{
  "manifest_version": 2,
  "name": "ReadPosition",
  "version": "1.0.0",
  "description": "Display read position in the page",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "applications": {
    "gecko": {
      "id": "readPosition@email.com"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "/content_script.js"
      ],
      "run_at": "document_end"
    }
  ]
}