doc about backporting to earlier releases

WIP.
> ... the X.0-current label is renamed to the patch release number and a new label X.0-next is created so if you look up the issues and PRs you directly see if they were backported and to what version.
The X.0-current label is renamed to the patch release number (X.0.Y), the current X.0-next label is renamed to X.0-current and a new X.0-next label is created, so if you look up the issues and PRs you directly see if they were backported and to what version..