Docs: update readme text to clarify handoffs (#208)

Clarifying that the handoff abstraction is implemented via Responses
tool calling

Author: dmitry-openai

Diff for: README.md

@@ -7,7 +7,7 @@ The OpenAI Agents SDK is a lightweight yet powerful framework for building multi
 ### Core concepts:
 
 1. [**Agents**](https://openai.github.io/openai-agents-python/agents): LLMs configured with instructions, tools, guardrails, and handoffs
-2. [**Handoffs**](https://openai.github.io/openai-agents-python/handoffs/): Allow agents to transfer control to other agents for specific tasks
+2. [**Handoffs**](https://openai.github.io/openai-agents-python/handoffs/): A specialized tool call used by the Agents SDK for transferring control between agents
 3. [**Guardrails**](https://openai.github.io/openai-agents-python/guardrails/): Configurable safety checks for input and output validation
 4. [**Tracing**](https://openai.github.io/openai-agents-python/tracing/): Built-in tracking of agent runs, allowing you to view, debug and optimize your workflows