[Doc] Add links to badges

This PR adds links to the following two badges:

1. RubyGems badge: A link to the `mcp` gem will be added. The repository name is `ruby-sdk`,
following the modelcontextprotocol org's naming convention. Since the repository name and
the gem name are different, the link will help make it clear which gem it refers to.
2. MIT license badge: This will link to the MIT license file in the repository.

NOTE: Separately from this PR, the copyright notice in the MIT license should probably be addressed.
It currently says `Copyright (c) 2025 TODO: Write your name`, so the TODO should be resolved:
https://github.com/modelcontextprotocol/ruby-sdk/blob/v0.1.0/LICENSE.txt#L3

Other language SDKs seem to list `Anthropic, PBC` as the copyright holder,
though I would defer that decision to someone more knowledgeable.

Author: koic

README.md

@@ -1,4 +1,4 @@
-# MCP Ruby SDK ![Gem Version](https://img.shields.io/gem/v/mcp) ![MIT licensed](https://img.shields.io/badge/license-MIT-green) [![CI](https://github.com/modelcontextprotocol/ruby-sdk/actions/workflows/ci.yml/badge.svg)](https://github.com/modelcontextprotocol/ruby-sdk/actions/workflows/ci.yml)
+# MCP Ruby SDK [![Gem Version](https://img.shields.io/gem/v/mcp)](https://rubygems.org/gems/mcp) [![MIT licensed](https://img.shields.io/badge/license-MIT-green)](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/LICENSE.txt) [![CI](https://github.com/modelcontextprotocol/ruby-sdk/actions/workflows/ci.yml/badge.svg)](https://github.com/modelcontextprotocol/ruby-sdk/actions/workflows/ci.yml)
 
 The official Ruby SDK for Model Context Protocol servers and clients.