Hay que ponerlo en la raíz del proyecto:
{
"repositories":
[
{
"type": "git",
"url": "https://url/to/git/project"
}
],
"require": {
"m4tthumphrey/php-gitlab-api": "dev-master",
"aws/aws-sdk-php": "3.*",
"jira-api": "dev-master"
},
"require-dev": {
"phpdocumentor/phpdocumentor": "2.*"
}
}