Added a comment to better demonstrate custom datatypes.

It is not obvious for a beginner (like me) to declare a new value of the temp datatype which can be either C (celsius) or F (fahrenheit). I think it would be better to demonstrate the declaration of such a datatype.

standard-ml.html.markdown

@@ -272,6 +272,9 @@ fun evenly_positioned_elems (odd::even::xs) = even::evenly_positioned_elems xs
 datatype temp =
       C of real
     | F of real
+    
+(*  Declaring a new C temp value...
+    val t: temp = C 45.0  *)
 
 fun temp_to_f t =
     case t of