doc: Update README quickstart to fix issue with LangGraph. (#56)

* doc: Update README quickstart to fix issue with LangGraph.

The quickstart earlier used the minimal example from LangChain's doc, and worked since earlier we returned `StructuredTool` from Toolbox SDK. Now with recent changes, we return a subclass of `BaseTool` which caused issues with the way this quickstart binds the tools. This PR addresses that by using the updated create_react_agent from LangGraph, and also pretty prints to better explain the output to the end-user.

* doc: Make the prompt more clearer in the README.

Author: anubhav756

README.md

@@ -36,19 +36,28 @@ applications, enabling advanced orchestration and interaction with GenAI models.
 
 ## Quickstart
 
-Here's a minimal example to get you started:
+Here's a minimal example to get you started using
+[LangGraph](https://langchain-ai.github.io/langgraph/reference/prebuilt/#langgraph.prebuilt.chat_agent_executor.create_react_agent):
 
 ```py
 from toolbox_langchain import ToolboxClient
 from langchain_google_vertexai import ChatVertexAI
+from langgraph.prebuilt import create_react_agent
 
 toolbox = ToolboxClient("http://127.0.0.1:5000")
 tools = toolbox.load_toolset()
 
 model = ChatVertexAI(model="gemini-1.5-pro-002")
-agent = model.bind_tools(tools)
-result = agent.invoke("How's the weather today?")
-print(result)
+agent = create_react_agent(model, tools)
+
+prompt = "How's the weather today?"
+
+for s in agent.stream({"messages": [("user", prompt)]}, stream_mode="values"):
+    message = s["messages"][-1]
+    if isinstance(message, tuple):
+        print(message)
+    else:
+        message.pretty_print()
 ```
 
 ## Installation