Add git submodules to git lesson

[pseyfert]

As discussed in LHCb Core Software 2019-04-10, LHCb's trainings/documentations lack good explanations of working with git submodules. I am unsure where this would best fit on the Xkit side of things given:

• dealing with them is at the moment probably needed rather late after joining (only for building entire "projects", probably not needed for analysis work) so it might not need to go in analysis-essentials
• submodules are a standard feature of git, so analysis-essentials seem right
• The issue doesn't occur when working with lb-dev git-lb-, i.e. doesn't fit the impactkit lb-dev lesson