doc: request better 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.

CONTRIBUTING.md

@@ -334,6 +334,10 @@ use `Refs:`.
    - `Refs: http://eslint.org/docs/rules/space-in-parens.html`
    - `Refs: https://github.com/nodejs/node/pull/3615`
 
+5. If your commit introduce a breaking change (`semver-major`), it should contain
+an explanation about the reason of the breaking change, which situation would trigger
+the breaking change and what is the exact change.
+
 Sample complete commit message:
 
 ```txt
@@ -625,6 +629,7 @@ Focus first on the most significant aspects of the change:
 1. Does this change make sense for Node.js?
 2. Does this change make Node.js better, even if only incrementally?
 3. Are there clear bugs or larger scale issues that need attending to?
+4. Is the commit message readable and correct? If it contains a breaking change is it clear enough?
 
 When changes are necessary, *request* them, do not *demand* them, and do not
 assume that the submitter already knows how to add a test or run a benchmark.