Add python version requirement to frontpage and readme (microsoft#5014)

Author: ekzhu

README.md

@@ -16,6 +16,8 @@
 
 ## Installation
 
+AutoGen requires **Python 3.10 or later**.
+
 ```bash
 # Install AgentChat and OpenAI client from Extensions
 pip install -U "autogen-agentchat" "autogen-ext[openai]"

python/packages/autogen-core/docs/src/index.md

@@ -109,7 +109,7 @@ Get Started
 
 </div>
 A programming framework for building conversational single and multi-agent applications.
-Built on Core.
+Built on Core. Requires Python 3.10+.
 
 ```python
 # pip install -U "autogen-agentchat" "autogen-ext[openai]"