message: Add API changes section
diff: @@ -226,6 +226,8 @@ Version 4.2.1 (development)
  - Added makefile rule to generate TAGS table for vi or Emacs users.
  +API changes
  +-----------
  - Added an abstract interface `mfem::FaceRestriction` for `H1FaceRestriction`
  and `L2FaceRestriction`.
  In order to conform with the semantic of `MultTranspose` in `mfem::Operator`,
  