I wanted to send a merge request to clarify some language in
README.Debian, but there doesn't seem to be any information on how
README.Debian is generated from README.Debian.m4. The most intuitive
way would be with a call like
   debian/rules debian/README.Debian

but there is no such Make target.

It's hard for a newcomer to find their way around a complex package
like gcc-defaults without an up-to-date README.source, and it seems
like this is something that should be documented there.