$ mkdir myproject
$ cd myproject
$ git init
Initialized empty Git repository in /Users/username/project/myproject/.git/

$ echo "Important code and data" > some_data.txt
$ git add some_data.txt
$ git commit -m "Initial commit on myproject"
[master (root-commit) 196bbbb] Initial commit on myproject
 1 file changed, 1 insertion(+)
 create mode 100644 some_data.txt

$ mkdir super
$ cd super
$ git init
Initialized empty Git repository in /Users/username/project/superproject/.git/

objects
├── info
└── pack

$ echo "This project will use ``myproject``" > README.txt
$ git add README.txt

objects
├── 9c
│   └── 0042144fc489d7b528ef186af49e78c2867f91 [43B]
├── info
└── pack

$ git commit -m "Initial commit on super"
[master (root-commit) 2326240] Initial commit on super
 1 file changed, 1 insertion(+)
 create mode 100644 README.txt

objects
├── 23
│   └── 262403a0b913d02219ead935dd1a85d3724a0d [139B]
├── 9c
│   └── 0042144fc489d7b528ef186af49e78c2867f91 [43B]
├── f1
│   └── 3a8c8331c76ac965c43b09d11ee2d72bb053c1 [55B]
├── info
└── pack

$ git submodule add ../myproject subproject
Cloning into 'subproject'...
done.

$ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	new file:   .gitmodules
	new file:   subproject