[!308] [doc] Document breaking change to ScalarProduct interface

Merge branch 'bug/document-scalarproduct-change' into 'master'

ref:core/dune-istl [!421] changed the signature of the methods dot() and
norm() in ScalarProduct, making them const. This breaks compilation of
existing user implementations, so let's document that change.

See merge request [!308]

  [!421]: gitlab.dune-project.org/NoneNone/merge_requests/421
  [!308]: gitlab.dune-project.org/core/dune-istl/merge_requests/308

CHANGELOG.md

@@ -37,6 +37,9 @@
 
 - Deprecated support for SuperLU 4.x. It will be removed after Dune 2.7.
 
+- The interface methods `dot()` and `norm()` of ScalarProduct are now `const`. You will
+  have to adjust the method signatures in your own scalar product implementations.
+
 # Release 2.6
 
 - `BDMatrix` objects can now be constructed and assigned from `std::initializer_list`.