[Changelog] Mention arbitrary refinements.

CHANGELOG.md

 # Release 2.6
 
 - The enum `GeometryType::BasicType` is deprecated, and will be removed after Dune 2.6.
+
+- `VirtualRefinement` and `Refinement` now support arbitrary refinements, not
+  just powers of two.  Whereever you where passing a parameter `int levels`
+  (now deprecated), you should now pass a parameter `RefinementIntervals
+  intervals`.  There are convenience functions `refinementIntervals(int
+  intervals)` and `refinementLevels(int levels)` to construct parameters of
+  type `RefinementIntervals`.