Add a changelog file.

diff --git a/doc/news/changes/incompatibilities/20201129Bangerth b/doc/news/changes/incompatibilities/20201129Bangerth
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+Removed: The file `affine_constraints.h` had a class called
+`IsBlockMatrix` that, however, also allowed to check whether its
+template argument is in fact a block sparsity pattern. The class was
+likely never intended to be in the public interface, and has been
+moved to internal::AffineConstraints::IsBlockMatrix and now also
+internal::AffineConstraints::IsBlockSparsityPattern.
+<br>
+(Wolfgang Bangerth, 2020/11/29)