NFD

Currently, CONTRIBUTING.md gives a good example of commit message title, which ended constant arguments about this between contributors and reviewers.
Many commit messages also have a body that (1) briefly explains the rationale of the commit (2) briefly describes the specifics of the change (3) lists other related changes not covered by the commit title in the form of “this commit also”.
However, there has been arguments on the formatting of these between contributors and reviewers. This issue is to provide a guideline on commit message bodies in CONTRIBUTING.md, to serve as a concrete reference in resolving such arguments.