docs: add config retrieval example to README

Add example showing how to access the current chat configuration and print
the model name, helping users understand how to retrieve configuration
values at runtime.

README.md

@@ -407,6 +407,10 @@ local prompts = chat.prompts()
 -- Get last copilot response (also can be used for integrations and custom keymaps)
 local response = chat.response()
 
+-- Retrieve current chat config
+local config = chat.config
+print(config.model)
+
 -- Pick a prompt using vim.ui.select
 local actions = require("CopilotChat.actions")