$ git commit -m "Added the submodule xxx (version: xxx) to the projet"
$ git push
Updating the submodules to a cetain hash (SHA-1) commit
$ git checkout commit-sha1 # SHA-1 related to the commit that will be the base for submodule
$ git commit -m "Udating submodule xxx to version xxx"
Extra Information
In order to add or remove a submodules, the command "$ git rm suddir" should be used to remove some submodule and the command "$ git submodule add <remote-url> <destination-folder>" can be replaced by the edition of .gitmodules files.