Point out that new stdlib modules need to be added to Makefile.pre.in

[encukou]

Makefile.pre.in is frequently forgotten, see python/cpython#13563, python/cpython#30311, python/cpython#31784.

The instructions for adding a new module to stdlib currently focus on what can be added rather than how to add something, so adding a “checklist step” might need some restructuring first.

[ezio-melotti]

Can we also detect this programmatically (e.g. in make patchcheck) in addition to documenting it?