First composer.json

Here is a composer.json. 

You will need to other stuff to make it usable by other people.

1) Go to packagist, register and add the package pointing to your github repo.

2) Go to Github, go to settings, click webhooks + services, add a service, select packagist, paste just your packagist username + token, (url isn't needed) and then click 'active'

3) Tag a version, wait a bit. Packagist should get updated with the first tagged version.

Technically people can work off "dev-master" but it's a pain in the arse.