MicroPython already requires contributors to implicitly sign-off on a set
of points, which are listed in CODECONVENTIONS.md.

This commit adjusts this wording to allow explicit sign-off using the git
"Signed-off-by:" feature.  There is no reference made to
https://developercertificate.org/

 because the project already has its own
version of this.

Signed-off-by: Damien George <damien@micropython.org>