docs: add beginner explanation for single agent example

README.md

@@ -108,6 +108,64 @@ adk web path/to/agents_dir
 - **Samples**: See `contributing/workflow_samples/` and
   `contributing/task_samples/` for workflow and task API examples.
 
+## 🏁 Feature Highlight
+
+> **Beginner note:**
+>
+> In ADK, an *agent* is a Python object that defines how your AI system behaves — including its instructions, the tools it can use, and how it responds to users. The example below shows the simplest way to define a single agent.
+
+### Define a single agent:
+
+```python
+from google.adk.agents import Agent
+from google.adk.tools import google_search
+
+root_agent = Agent(
+    name="search_assistant",
+    model="gemini-2.5-flash", # Or your preferred Gemini model
+    instruction="You are a helpful assistant. Answer user questions using Google Search when needed.",
+    description="An assistant that can search the web.",
+    tools=[google_search]
+)
+```
+
+### Define a multi-agent system:
+
+Define a multi-agent system with coordinator agent, greeter agent, and task execution agent. Then ADK engine and the model will guide the agents works together to accomplish the task.
+
+```python
+from google.adk.agents import LlmAgent, BaseAgent
+
+# Define individual agents
+greeter = LlmAgent(name="greeter", model="gemini-2.5-flash", ...)
+task_executor = LlmAgent(name="task_executor", model="gemini-2.5-flash", ...)
+
+# Create parent agent and assign children via sub_agents
+coordinator = LlmAgent(
+    name="Coordinator",
+    model="gemini-2.5-flash",
+    description="I coordinate greetings and tasks.",
+    sub_agents=[ # Assign sub_agents here
+        greeter,
+        task_executor
+    ]
+)
+```
+
+### Development UI
+
+A built-in development UI to help you test, evaluate, debug, and showcase your agent(s).
+
+<img src="https://raw.githubusercontent.com/google/adk-python/main/assets/adk-web-dev-ui-function-call.png"/>
+
+###  Evaluate Agents
+
+```bash
+adk eval \
+    samples_for_testing/hello_world \
+    samples_for_testing/hello_world/hello_world_eval_set_001.evalset.json
+```
+
 ## 🤝 Contributing
 
 See [CONTRIBUTING.md](CONTRIBUTING.md) for details.