Added beginner-friendly Ruby tutorial

Added a beginner-friendly Ruby tutorial - a step-by-step walkthrough on how to build a Rails application that lets users post information about sharks and their behavior.

Author: samcoren

README.md

@@ -505,6 +505,7 @@ To get started, simply fork this repo. Please refer to [CONTRIBUTING.md](CONTRIB
 - [The Ruby on Rails Tutorial](https://www.railstutorial.org/book)
 - [Build Instagram From Scratch with Ruby on Rails](https://www.dropbox.com/s/9vq430e9s3q7pu8/Let%27s%20Build%20Instagram%20with%20Ruby%20on%20Rails%20-%20Free%20Edition.pdf?dl=0)
 - [Build a Social Network using Rails](https://medium.com/rails-ember-beyond/how-to-build-a-social-network-using-rails-eb31da569233)
+- [How To Build a Ruby on Rails Application](https://www.digitalocean.com/community/tutorials/how-to-build-a-ruby-on-rails-application)
 
 ## Haskell: