Update README with maintenance and resource information (#7067)

Author: ekzhu

README.md

@@ -15,6 +15,10 @@
 
 **AutoGen** is a framework for creating multi-agent AI applications that can act autonomously or work alongside humans.
 
+> **Important:** if you are new to AutoGen, please checkout [Microsoft Agent Framework](https://github.com/microsoft/agent-framework).
+> AutoGen will still be maintained and continue to receive bug fixes and critical security patches.
+> Read our [announcement](https://github.com/microsoft/autogen/discussions/7066).
+
 ## Installation
 
 AutoGen requires **Python 3.10 or later**.