Link to readme on failed install of project/dir empty

[markshust]

This error is too prevalent, and is causing too many GitHub issues to be filed:

Project directory "/var/www/html/." is not empty error

It is also represented by:

chmod: cannot access 'bin/magento': No such file or directory

What I'd like is that on bin/start, detect the presence of either of these errors, then show a message to the user that they have a failed install, and to link to https://github.com/markshust/docker-magento#install-fails-because-project-directory-is-not-empty for more information.