Add note to README regarding the plugin assets

[pboivin]

The command should run after the plugins are registered to ensure that the assets are published

[what-the-diff]

PR Summary

• Update to README.md File
  This change has included a new section in the README.md file, providing instructions to publish registered plugin assets. This is done through the PHP Artisan tool with the specific 'filament:assets' command. It essentially helps users better utilize plugin assets in the project, improving the asset management process.