add changelogs from release to master

CHANGELOG.md

 # Master (will become release 2.7)
+
+# Release 2.6
+
+- `BDMatrix` objects can now be constructed and assigned from `std::initializer_list`.
+
+- `BDMatrix` and `BTDMatrix` now implement the `setSize` method, which allows to
+  resize existing matrix objects.
+