Examine source code of Learn the shortcuts

{
  "manifest_version": 2,
  "name": "Learn the shortcuts",
  "version": "0.6.2",
  "description": "Learn keyboard shortcuts painlessly for Gmail, Trello, Github & Google Calendar",
  "short_name": "LTS",
  "icons": {
    "16": "icons/logo-16.png",
    "48": "icons/logo-48.png",
    "128": "icons/logo-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "shortcuts.js",
        "gmail.js"
      ]
    },
    {
      "matches": [
        "https://github.com/*"
      ],
      "js": [
        "shortcuts.js",
        "github.js"
      ]
    },
    {
      "matches": [
        "https://trello.com/*"
      ],
      "js": [
        "shortcuts.js",
        "trello.js"
      ]
    },
    {
      "matches": [
        "https://calendar.google.com/*"
      ],
      "js": [
        "shortcuts.js",
        "google-calendar.js"
      ]
    }
  ],
  "permissions": [],
  "web_accessible_resources": [
    "shortcuts.js",
    "gmail.js",
    "github.js",
    "trello.js",
    "google-calendar.js"
  ]
}