Composer is a new dependency manager for PHP. It allows you to specify dependencies on a per-project basis. It takes lots of inspiration from NPM and ruby's bundler.
All you need to support composer is a composer.json file. In order to allow easy installation, the repository needs to be added to packagist, which is the standard repository for composer. Packagist will fetch all the versions from your github repository tags.
Once it has been added, adding {PROJECT} to a project will be as easy as creating this composer.json file in the project's directory:
{
"require": {
"{VENDOR}/{PROJECT}": "{VERSION}"
}