Tutorial on how to use any submodule in any repository like here the SimplifiedGmailApi.
(If you want to learn more visit this site)
If you just clone the repository there will be an empty folder where the submodule is normally located.
To get it's content use either this at the git clone process:
git clone --recursive <project url>
Or if you already cloned the repository use:
git submodule update --init --recursive
Have fun programming! 😃
PS: If you want to use this submodule do not forget to activate the API in the main project script
You used a newer version of the submodule and want to update the submodule link not only locally but on GitHub to this version:
git submodule update --remote --merge
Now the submodule file points to the current path the submodule is right now.
With this you can add a submodule yourself to a new directory of your current project:
git submodule add <project you want to use as submodule url> <directory name>