git submodule
  sync   - update .git/config based on .gitmodules
  init
  update
    --remote
  add <url> [folder]

git clone --recursive <url>

!after switching main repo to a new feature-branch, don't forget to do:
    git submodule update

2013
http://blogs.atlassian.com/2013/03/git-submodules-workflows-tips/
https://medium.com/@porteneuve/mastering-git-submodules-34c65e940407

auto-access
https://stackoverflow.com/questions/40841882/automatically-access-git-submodules-via-ssh-or-https