Add instructions to update the vercel-submodule-workaround

[trebeljahr]

For me when following the approach as outlined in the Readme it didn't work and I had to update a few things in the setup script.

Specifically, things outlined in this comment:
https://github.com/orgs/vercel/discussions/44#discussioncomment-3391318

I.e.
Instead of:

git fetch --depth=1 origin $COMMIT # fetch only the required version
git checkout $COMMIT # checkout on the right commit

I used:

git fetch origin
git checkout main 
npm install

and I also updated the references for the variables at the beginning of the script accordingly

# github submodule repo address without https:// prefix
SUBMODULE_GITHUB=path to your own, don't forget the .git bit at the end

# .gitmodules submodule path
SUBMODULE_PATH=this can be copied from the gitmodules file

[trebeljahr]

maybe the readme can be updated to make it clearer that there is extra setup needed and people don't go "ugh, this doesn't work" ^^