add existing checks to repo as described in the docs

[leonhusmann]

This PR completes #788 as described in the documentation.

The documentation example wasn't implemented yet.

$checks = $client
    ->api('repo')
    ->checks()
    ->create('NimbleCI', 'docker-web-tester-behat', $params);

[acrobat]

Thanks @leonhusmann! And congrats on your first contribution! 🎉