Add MCP C# SDK Documentation link to top-level README

Co-authored-by: jeffhandley <1031940+jeffhandley@users.noreply.github.com>

Author: Copilot

README.md

@@ -26,7 +26,8 @@ The Model Context Protocol (MCP) is an open protocol that standardizes how appli
 
 For more information about MCP:
 
-- [Official Documentation](https://modelcontextprotocol.io/)
+- [Official MCP Documentation](https://modelcontextprotocol.io/)
+- [MCP C# SDK Documentation](https://modelcontextprotocol.github.io/csharp-sdk/)
 - [Protocol Specification](https://modelcontextprotocol.io/specification/)
 - [GitHub Organization](https://github.com/modelcontextprotocol)