There are some usecases where you want to integrate arbitrary libraries into your system. One example I stumbled upon was some NPM library which stored some general taxonomy mappings in json files. In Drupal we wanted to synchronize these information.

Therefore we are using this small little bit in composer.json:

{
    "repositories": [
        {
           "type": "package",
           "package": {
                "name": "namespace/name",
                "version": "dev-master",
                "source": {
                    "url": "https://github.com/example/example-repo.git",
                    "type": "git",
                    "reference": "master"
                }
            }
        }
    ]
}

Once this is done you can use:

composer require namespace/name

And well this is all you need to know. Pratically you basically specify the composer.json information inside another composer.json file.