doc: outline commit message for breaking changes

Add informations to CONTRIBUTING.md requesting the committers
and especially the one working on breaking changes to write
proper commits messages containing the reason behind the
breaking changes, the case during which it can be triggered,
and a description of the breaking change in itself.

PR-URL: https://github.com/nodejs/node/pull/16846


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>