Add entry about reference element changes

CHANGELOG.md

@@ -8,3 +8,63 @@
   intervals`.  There are convenience functions `refinementIntervals(int
   intervals)` and `refinementLevels(int levels)` to construct parameters of
   type `RefinementIntervals`.
+  - The class and most of its methods are now `constexpr`.
+
+  - There are new singletons and factory functions in the namespace `Dune::GeometryTypes`. These
+    are now the official way to obtain a `GeometryType`.
+
+  - `GeometryType::BasicType` and the assorted constructor have been deprecated and will be removed
+    after the release of DUNE 2.6.
+
+  - The assorted member functions `GeometryType::make...()` have been deprecated and will be removed
+    after the release of DUNE 2.6.
+
+- The reference element interface has had a substantial overhaul that can break backwards
+  compatibility in some corner cases.
+
+  - `ReferenceElement` has value semantics now: You should store instances by value and can freely
+    copy them around. Doing so is not more expensive than storing a const reference.
+
+  - As a consequence of value semantics, `ReferenceElement` is default constructible now. A default
+    constructed `ReferenceElement` may only be assigned another `ReferenceElement`; all other
+    oeprations cause undefined behavior. Moreover, instances are now comparable and hashable to
+    allow storing them in maps.
+
+  - We have added code that tries to warn you if you are still storing a `ReferenceElement` by const
+    reference; please update all those occurences.
+
+  - The meaning of `Dune::ReferenceElement` has changed. It is not a type anymore, but an alias
+    template that looks up the correct implementation for the given template arguments. For now,
+    there is only a single implementation, but we expect people to come up with additional
+    implementations in the future. For this reason, the syntax `Dune::ReferenceElement<ctype,dim>`
+    is deprecated and will cause compilation failures in the future. If you still need access to
+    that type, use `typename Dune::ReferenceElements<ctype,dim>::ReferenceElement` instead.
+
+  - You can now directly obtain a reference element for a given geometry using the free function
+    `referenceElement(geometry)`. This function should be called without any namespace qualifiers to
+    enable ADL and you should normally capture the return value of the function using `auto`, but if
+    you need to explicitely access the type, this is also available as
+    `Dune::ReferenceElement<Geometry>`.
+
+  In short: If you can, use the following idiom to obtain a reference element for a geometry:
+  ```c++
+  auto ref_el = referenceElement(geometry);
+  ```
+
+  The change to the meaning of `Dune::ReferenceElement` can break compilation if you have function
+  overloads that partially specialize on it, e.g.
+  ```c++
+  template<typename ctype, int dim>
+  void f(const Dune::ReferenceElement<ctype,dim> ref_el)
+  {}
+  ```
+  Normally, you can just simplify this to the following code that also shows how to extract the
+  missing template parameters:
+  ```c++
+  template<typename RefEl>
+  void f(const RefEl ref_el)
+  {
+    using ctype = typename RefEl::CoordinateField;
+    constexpr auto dim = RefEl::dimension;
+  }
+  ```