Add drush with composer for site-wide use

I have a question about adding drush for site-wide use thru the composer.json.

i added the latest version with composer "composer require drush/drush:dev-master".
after that i try to use drush in the docroot folder, but get an error:
Unable to load autoload.php. Run composer install to fetch dependencies and write this file (http://docs.drush.org/en/master/install-alternative/). Or if you prefer, use the drush.phar which already has dependencies included (http://docs.drush.org/en/master/install).

iam not shure it this depends on my server config, or on my way to add drush.

maybe someone can help me, please.