add changelogs from release to master

CHANGELOG.md

 # Master (will become release 2.7)
+
+# Release 2.6
+
+- New class `IntegralRange<integral_type>` and free standing function
+  `range` added, providing a feature similar to Python's `range` function:
+  ```
+    for (const auto &i : range(5,10))
+  ```
+  See core/dune-common!325
+
+- `Dune::array` was deprecated, use `std::array` from <array> instead.
+   Instead of `Dune::make_array`, use `Dune::Std::make_array`
+   from dune/common/std/make_array.hh
+   and instead of `Dune::fill_array` use `Dune::filledArray`
+   from dune/common/filledarray.hh.`
+
+    See core/dune-common!359
+
+- The `DUNE_VERSION...` macros are deprecated use the new macros
+  `DUNE_VERSION_GT`, `DUNE_VERSION_GTE`, `DUNE_VERSION_LTE`, and
+  `DUNE_VERSION_LT` instead.
+
+    See core/dune-common!329
+
+- Added some additional fallback implementation to C++17 features:
+  (e.g. `optional`, `conjunction`, `disjunction`)
+
+- `makeVirtualFunction`:
+  allows to easily convert any function object (e.g. lambda) to a `VirtualFunction`
+
+    See core/dune-common!282
+
+- `FieldMatrix` now has experimental support for SIMD types from
+  [Vc](https://github.com/VcDevel/Vc) as field types.
+
+    See core/dune-common!121