Update README: Document Push Notification Support in Chrome Extension

[GAURAVCHAWLA1808]

• Added details on integrating push notifications using the Push API.
• Documented changes in background.js, popup.js, and popup.html for handling push events.
• Included steps for testing push notifications and debugging service workers.
• Ensured the guide aligns with the latest Chrome Extension best practices.

This update improves documentation for developers integrating push notifications into their Chrome extensions.