Refer to useful related modules for REST apis

[nichtich]

The documentation should better explain how to mix the module with other modules to create REST APIs

• Plack::Middleware::Negotiate to content negotiation
• Plack::Middleware::MethodOverride to support X-HTTP-Method-Override
• Plack::Middleware::Auth::AccessToken and other for authentification
• Plack::Middleware::Throttle or Plack::Middleware::Throttle::Lite for rate limiting
• Plack::Middleware::CrossOrigin for CORS

This should also make clear why Plack::Middleware::REST is not yet another uber-framework to do everything in one module.