Examine source code of GitHub Rulers

{
  "manifest_version": 2,
  "name": "GitHub Rulers",
  "version": "1.3",
  "description": "Displays 0, 80 & 120 character stop in the GitHub web editor",
  "icons": {
    "48": "48.png",
    "96": "96.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "*://*.github.com/*"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e9c1c993-2539-4d68-8165-24134bd2fa92}"
    }
  }
}