Added Sampling Example to README

[RishiNandha]

Motivation and Context

I had opened #697 earlier today when I was trying to find out how to send sampling requests to MCP Clients from an MCP Server made using our SDK. There is no explicit mention of "Sampling" in the README while the SDK supports this function.

How Has This Been Tested?

I have tested the example I've added into README to work. The received result in the attached example is of the below format when used with VS Code's Github Co-Pilot as the client:

{
    "model" : "github.copilot-chat/gpt-4.1",
    "role" : "assistant",
    "content": {
        "type" : "text",
        "text" : "text response"
    }
}

Breaking Changes

No Breakages

Types of changes

• [ x ] Documentation update

Checklist

• [ x ] I have read the MCP Documentation
• [ x ] My code follows the repository's style guidelines
• [ NA ] New and existing tests pass locally
• [ NA ] I have added appropriate error handling
• [ x ] I have added or updated documentation as needed