add Ruby subsection

Author: tuvtran

README.md

@@ -180,6 +180,10 @@ To get started, simply fork this repo. Please refer to [CONTRIBUTING.md](CONTRIB
 
 ## Ruby:
 
+* [Build a Network Stack with Ruby](https://medium.com/geckoboard-under-the-hood/how-to-build-a-network-stack-in-ruby-f73aeb1b661b)
+
+### Ruby on Rails:
+
 * [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 CMS with Ruby on Rails](https://pchm.co/posts/tutorial-how-to-build-a-cms-in-ruby-on-rails)
 * [Build a Social Network using Rails](https://medium.com/rails-ember-beyond/how-to-build-a-social-network-using-rails-eb31da569233)