Add “pull/update submodules” steps for private contributors (SSH) in repos that use submodules

[ronhedwigzape]

For private contributors (direct repo access) who are not yet familiar with Git submodules / recursive updates, please add a quick tip on how to keep submodules in sync after pulling main. Pulling the parent repo alone won’t update submodule commits and can cause mismatches/build issues.

Proposed README addition (for repos with submodules):

git checkout main
git pull origin main
git submodule update --init --recursive