added comment for WolframAlpha input

tikz/hypersurface-order-2/hypersurface-order-2.tex

@@ -42,6 +42,8 @@
         %xticklabels={,,},
         %yticklabels={,,}
     ]
+      % if you want to use Wolfram|Alpha:
+      % Plot3D[x*x-4*x*y+y*y+1, {x, -5, 5}, {y, -5, 5}]
       \addplot3[surf] {x*x-4*x*y+y*y+1};
     \end{axis} 
 \end{tikzpicture}