doc: Add instructions for git submodules

[RaulTrombin]

Solves:
#534

[tiagolobocastro]

What about the previous suggestion of adding the git command to the make prepare command? Do you think it's better to keep as is?

[RaulTrombin]

What about the previous suggestion of adding the git command to the make prepare command? Do you think it's better to keep as is?

This would also resolve the issue. I tested it on a clean clone of the repo, and it seems fine. May I go ahead and add this commit as well?

[tiagolobocastro]

What about the previous suggestion of adding the git command to the make prepare command? Do you think it's better to keep as is?

This would also resolve the issue. I tested it on a clean clone of the repo, and it seems fine. May I go ahead and add this commit as well?

Yeah I think so if you don't mind

[RaulTrombin]

What about the previous suggestion of adding the git command to the make prepare command? Do you think it's better to keep as is?

This would also resolve the issue. I tested it on a clean clone of the repo, and it seems fine. May I go ahead and add this commit as well?

Yeah I think so if you don't mind

did it,
removed the unnecessary aditional command if user skips submodule, as the 'make prepare' will fix it.