Add Concurrency section

[iJackUA]

Celluloid Concurrent-ruby

General Thread / Fibers info links

https://www.codeotaku.com/journal/2018-11/fibers-are-the-right-solution/index

Improve description of the different app servers workflow and why threads can cause issues (before_fork etc.)

or writing own rack server from scratch https://blog.appsignal.com/2017/01/24/ruby-magic-building-a-ruby-http-server-part-2-running-a-rails-app.html