Examine source code of Custom Gaintime extension

{
  "name": "Custom Gaintime extension",
  "description": "Example extension showing use of the InboxSDK adding a sidebar to a Gmail thread",
  "version": "1.0.0",
  "icons": {
    "16": "./favicon.png",
    "48": "./favicon48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*",
        "https://inbox.google.com/*"
      ],
      "js": [
        "inboxsdk.js",
        "content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "web_accessible_resources": [
    "favicon.png"
  ],
  "permissions": [
    "https://mail.google.com/",
    "https://inbox.google.com/"
  ],
  "browser_action": {
    "default_icon": "./favicon48.png"
  },
  "manifest_version": 2,
  "browser_specific_settings": {
    "gecko": {
      "id": "{f596e473-712c-4cce-8fee-2e55906bbb9b}"
    }
  }
}