Update configuration documentation for Alpha Vantage data vendor

Add data vendor configuration examples in README and main.py showing how to configure Alpha Vantage as the primary data provider. Update documentation to reflect the current default behavior of using Alpha Vantage for real-time market data access.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

main.py

@@ -9,6 +9,14 @@ config["deep_think_llm"] = "gemini-2.0-flash"  # Use a different model
 config["quick_think_llm"] = "gemini-2.0-flash"  # Use a different model
 config["max_debate_rounds"] = 1  # Increase debate rounds
 
+# Configure data vendors (default uses Alpha Vantage for real-time data)
+config["data_vendors"] = {
+    "core_stock_apis": "alpha_vantage",      # Options: alpha_vantage, yahoo_finance, local
+    "technical_indicators": "alpha_vantage", # Options: alpha_vantage, yahoo_finance, local
+    "fundamental_data": "alpha_vantage",     # Options: alpha_vantage, openai, local
+    "news_data": "alpha_vantage",            # Options: alpha_vantage, openai, google, local
+}
+
 # Initialize with custom config
 ta = TradingAgentsGraph(debug=True, config=config)