Use GitHub note admonition for clients note

Author: localden

README.md

@@ -80,6 +80,7 @@ Once installed, verify by asking your agent _"What skills do you have?"_ — you
   <a href="https://www.mcpjam.com/"><img src="https://img.shields.io/badge/MCPJam-client-8B5CF6" alt="MCPJam"></a>
 </p>
 
+> [!NOTE]
 > MCP Apps is an extension to the [core MCP specification](https://modelcontextprotocol.io/specification). Host support varies — see the [clients page](https://modelcontextprotocol.io/clients) for the full list.
 
 ## Why MCP Apps?