Add cookbook reference to main README.md

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Author: Copilot

README.md

@@ -105,6 +105,7 @@ jbang https://github.com/copilot-community-sdk/copilot-sdk-java/blob/latest/jban
 - [Getting Started](https://copilot-community-sdk.github.io/copilot-sdk-java/documentation.html)
 - [Javadoc API Reference](https://copilot-community-sdk.github.io/copilot-sdk-java/apidocs/)
 - [MCP Servers Integration](https://copilot-community-sdk.github.io/copilot-sdk-java/mcp.html)
+- [Cookbook](cookbook/) — Practical recipes for common use cases
 
 ## Projects Using This SDK