Update README.md: Explain how to deal with updates in sub-projects dependencies

[gg-mmill]

Hello, thanks a lot for this example repo.

This issue bugged me for some time (until I realized that pdm lock indeed re-generated the lock file correctly).

I first thought there was a deeper bug, that prevented taking into account modifications in the dependencies of the sub-projects.

[sinogermany]

@gg-mmill I'd suggest adding this into README.md too:

ls packages | xargs -I % sh -c "pdm update -p packages/%"

or create a wrapper shell script, e.g. pdm-sub-pkgs.sh :

ls packages | xargs -I % sh -c "pdm ${@} -p packages/%"

Then you can do ./pdm-sub-pkgs.sh update