COMMITSTYLE.md: Add addition and removal of maintainership

I would've preferred "remove maintainership" over "drop maintainership", but the latter is the status quo.

See https://gitlab.alpinelinux.org/alpine/aports/-/commits/master?search=remove+maintainership vs https://gitlab.alpinelinux.org/alpine/aports/-/commits/master?search=drop+maintainership.