Say you want the contents of a given folder in your project to point to a different Git repo's (top folder).
Create a file called .gitmodules in your projects top folder:
[submodule "src/main/other"]
path = src/main/other
url = ssh://git@git.example.com/other.git
Then do:
$ git submodule init
$ git submodule update
The contents of your project's folder src/main/other is now
"linked" to the Git repo called other.
When you later want to update the submodule, you'll need to switch to its desired branch and then pull in the changes:
$ cd src/main/other
$ git checkout master
$ git pull
