Examine source code of Remember to Smile

{
  "name": "Remember to Smile",
  "description": "This extension will remind you to consider visiting smile.amazon.com, smile.amazon.co.uk, or smile.amazon.de instead of www.amazon.com, www.amazon.co.uk, or www.amazon.de, respectively, so that a portion of your eligible purchases' price will go towards a charitable organization of your choice. It stores no information about you or your browsing history.",
  "version": "1.0.2",
  "permissions": [],
  "homepage_url": "https://github.com/awzuelsdorf/remember_to_smile",
  "content_scripts": [
    {
      "matches": [
        "https://*/*"
      ],
      "js": [
        "remember_to_smile.js"
      ]
    }
  ],
  "manifest_version": 2,
  "icons": {
    "96": "icon96x96.png"
  }
}