It's hard for anyone to deny just how wonderful an investment Chipotle Mexican Grill (CMG -2.14%) has been. Its shares have catapulted higher in the last few years thanks to a strong financial performance.The fact that Chipotle has proven to be an outstanding business that has compounded shareholder capital means that it should be on your investing radar. However, it's best to know about one key warning before buying this unstoppable restaurant stock.Chipotle's sky-high valuationIn the past five years, Chip ...