feat: add deprecation notice to stream-chat-php README

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mogita

README.md

@@ -1,3 +1,9 @@
+> **Note:** [`getstream-php`](https://github.com/GetStream/getstream-php) is the new full-product PHP SDK for Stream, covering Chat, Video, Moderation, and Feeds with long-term support.
+>
+> **`stream-chat-php` is now in maintenance mode.** It will continue to receive critical fixes and requested features, so it is safe to keep using. However, we encourage existing users to migrate when convenient and strongly recommend that new projects start with `getstream-php`.
+>
+> Already using `stream-chat-php`? See the [Migration Guide](docs/migration-to-getstream-php/README.md) for side-by-side examples covering every use case.
+
 # Official PHP SDK for [Stream Chat](https://getstream.io/chat/)
 
 [![build](https://github.com/GetStream/stream-chat-php/workflows/build/badge.svg)](https://github.com/GetStream/stream-chat-php/actions) [![Latest Stable Version](https://poser.pugx.org/get-stream/stream-chat/v/stable)](https://packagist.org/packages/get-stream/stream-chat)