Add guidelines for working with submodules

[gabryelreyes]

Some points could be addressed by the working instructions relevant to submodules and their integration:

• PRs in submodules should be merged in the correct order if several repositories depend on each other
• repository dependencies should be documented in the PRs so that the integrator knows of other repositories that shall be integrated simultaneously
• The repositories modified for a feature should be merged only if:
  • All repositories are ready to be merged
  • OR merging the PR and integrating the repository does not block any other feature or pipeline.
  • OR the repository does not have any dependency