docs: add beginner explanation for single agent example

Merge google#4099

### Description of the change

**Problem:**
New users may find it unclear what an “agent” represents in ADK before encountering the first code example in the README.

**Solution:**
Added a short beginner-friendly explanation before the single-agent example to clarify what an ADK agent is and what the example demonstrates. This improves readability and onboarding without changing any existing behavior or code.

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### Testing Plan

This change is documentation-only and does not affect runtime behavior. No tests were required or run.

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### Checklist

- [x] I have read the CONTRIBUTING.md document.
- [x] I have performed a self-review of my own change.
- [ ] I have commented my code, particularly in hard-to-understand areas. (Not applicable – documentation only)
- [ ] I have added tests that prove my fix is effective or that my feature works. (Not applicable – documentation only)
- [ ] New and existing unit tests pass locally with my changes. (Not applicable – documentation only)
- [ ] I have manually tested my changes end-to-end. (Not applicable – documentation only)
- [ ] Any dependent changes have been merged and published in downstream modules. (Not applicable)

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### Additional context

This change is intended to improve the onboarding experience for users exploring ADK for the first time.

Co-authored-by: Shangjie Chen <deanchen@google.com>
COPYBARA_INTEGRATE_REVIEW=google#4099 from yukthagangadhari5:docs-beginner-note ec2e677
PiperOrigin-RevId: 932731929

Author: 2 people

README.md

@@ -59,6 +59,10 @@ The release cadence is roughly bi-weekly.
 
 ## Quick Start
 
+> **Beginner Note:** ADK applications are built using two main classes:
+> **`Agent`** (defines an AI's instructions, tools, and behavior) and
+> **`Workflow`** (orchestrates agents and tasks in a graph-based flow).
+
 ### Agent
 
 ```python