Simply try below command after you have added your submodules.

for git v 1.8.x

git submodule update --init --recursive --remote

for v1.7.x:

git submodule update --init --recursive or git pull --recurse-submodules

for v1.6.x

git submodule foreach git pull origin master

to check your git version.

git version

from https://stackoverflow.com/a/43536170