doc: Update the CONTRIBUTING.md to follow Conventional Commits

Following this convention will let NuttX commits to be easily
parsed by standard tools: https://www.conventionalcommits.org/en/v1.0.0/

Signed-off-by: Alan C. Assis <[email protected]>

Author: acassis

CONTRIBUTING.md

@@ -285,8 +285,11 @@ We avoid breaking changes unless absolutely necessary and unavoidable
  8. Change must be well documented (build / runtime test logs, pr, git
     commit, documentation, release notes, etc) with clear notes on how to
     fix the introduced problems.
- 9. Breaking Change must be clearly marked with a `[BREAKING]` tag in the
-    git commit topic and PR title that will propagate to Release Notes.
+ 9. Breaking Change must be clearly identified with inclusion of the an
+    exclamation mark `!` as first character of the commit title and a
+    `BREAKING CHANGE:` in the git commit log message, followed by a short
+    comment about way user will use it currently. This change description
+    will be used to create the Release Notes.
 
 See: https://github.com/apache/nuttx/blob/master/INVIOLABLES.md