Add note about minimum stability to README (#21)

* Add note about minimum stability to README

* Remove unnecessary blank line.

Author: chr-hertel

README.md

@@ -51,6 +51,12 @@ Examples working
 composer require mcp/sdk
 ```
 
+Since this package has no tagged releases yet, it is required to extend your `composer.json`:
+```json
+    "minimum-stability": "dev",
+    "prefer-stable": true,
+```
+
 ## ⚡ Quick Start: Stdio Server with Discovery
 
 This example demonstrates the most common usage pattern - a `stdio` server using attribute discovery.