docs: update README with JSON example for notifications

[appleboy]

• Add JSON example for sending notifications with analytics label in README.md

Label associated with the message's analytics data.

{
  "notifications": [
    {
      "tokens": ["token_a", "token_b"],
      "platform": 2,
      "message": "Hello World Android!",
      "title": "You got message",
      "fcm_options": {
        "analytics_label": "example"
      }
    }
  ]
}

fix #746
fix #444