Add information about delay on "rbenv install 2.5.1" step

[polnetwork]

The setup is explained very clear, the only point where I'v doubt is on the "rbenv install 2.5.1" command because this could take a while and there's no visual information about the progress. Maybe we can explain on the documentation that. Thank's for all your work.

[microstudi]

Good Idea, I'll incorporate that. I need to follow the guide myself to be sure thinks are accurate!