Add repositorie in composer.json :
"repositories": [
{
"type": "composer",
"url": "https://packages.drupal.org/8"
},
{
"type": "git",
"url": "https://github.com/dom18fr/cleverdrop"
}
],
Add requirement in composer.json
"require": {
"drupal/core": "~8.0",
"drupal/cleverdrop": "dev-master"
}
OR
composer require drupal/cleverdrop:dev-master