add news bullet

NEWS.md

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 # scales (development version)
+* Range training now supports custom classes. Continuous classes require a
+  `range()` method that returns numeric values. Discrete classes require a
+  `levels()` method (and optionally a `droplevels()` method) (#480).
 * New `label_glue()` labelling function for interpolated strings (#457).
 * `fullseq()` and by extension `breaks_width()` can now deal with unsorted 
   ranges (#435).