Examine source code of Reading Time

{
  "manifest_version": 2,
  "name": "Reading Time",
  "version": "1.8",
  "description": "Displays the estimated reading time for a website in a small popup element.",
  "homepage_url": "https://github.com/scripts-richard/reading_time",
  "icons": {
    "48": "icons/reading_time-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "exclude_matches": [
        "*://*.reddit.com/*",
        "*://*.twitter.com/*",
        "*://*.google.com/*",
        "*://*.youtube.com/*",
        "*://*.bing.com/*",
        "*://*.duckduckgo.com/*",
        "*://*.amazon.com/*"
      ],
      "js": [
        "./reading_time.js"
      ],
      "css": [
        "./reading_time.css"
      ],
      "run_at": "document_end"
    }
  ],
  "permissions": [
    "storage"
  ],
  "options_ui": {
    "page": "options.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{df990922-ed8a-439b-9c48-e2179e5d0ee2}"
    }
  }
}