Make package readmes slightly less empty (microsoft#4961)

* Make package readmes slightly less empty

* Update python/packages/autogen-ext/README.md

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

Author: jackgerrits

python/packages/autogen-agentchat/README.md

@@ -1,3 +1,12 @@
 # AutoGen AgentChat
 
 - [Documentation](https://microsoft.github.io/autogen/stable/user-guide/agentchat-user-guide/index.html)
+
+AgentChat is a high-level API for building multi-agent applications.
+It is built on top of the [`autogen-core`](https://microsoft.github.io/autogen/stable/user-guide/core-user-guide/index.html) package.
+For beginner users, AgentChat is the recommended starting point.
+For advanced users, [`autogen-core`](https://microsoft.github.io/autogen/stable/user-guide/core-user-guide/index.html)'s event-driven
+programming model provides more flexibility and control over the underlying components.
+
+AgentChat provides intuitive defaults, such as **Agents** with preset
+behaviors and **Teams** with predefined [multi-agent design patterns](https://microsoft.github.io/autogen/stable/user-guide/core-user-guide/design-patterns/intro.html).

python/packages/autogen-core/README.md

@@ -1,3 +1,5 @@
 # AutoGen Core
 
 - [Documentation](https://microsoft.github.io/autogen/stable/user-guide/core-user-guide/index.html)
+
+AutoGen core offers an easy way to quickly build event-driven, distributed, scalable, resilient AI agent systems. Agents are developed by using the [Actor model](https://en.wikipedia.org/wiki/Actor_model). You can build and run your agent system locally and easily move to a distributed system in the cloud when you are ready.

python/packages/autogen-ext/README.md

@@ -1,3 +1,5 @@
-# autogen-ext
+# AutoGen Extensions
 
-[Documentation](https://microsoft.github.io/autogen/stable/user-guide/extensions-user-guide/index.html)
+- [Documentation](https://microsoft.github.io/autogen/stable/user-guide/extensions-user-guide/index.html)
+
+AutoGen is designed to be extensible. The `autogen-ext` package contains many different component implementations maintained by the AutoGen project. However, we strongly encourage others to build their own components and publish them as part of the ecosytem.