Add the asciidoctor installation steps to Readme

[charlottebrf]

Please remove items in the right column:

Type	Idea
Screenshot included?	no
Description

1. Make the project more inclusive for beginners working on open source projects by adding asciidoctor installation steps to Readme: http://asciidoctor.org/
2. Be clear in the Readme that users only need to do: (a) $ gem update asciidoctor & $ gem install --pre asciidoctor-pdf (if memory is correct) - as I beginner when I first looked at documentation I was unsure of if I needed to go through rest of steps which were only relevant to using ascii for setting up a new project
3. Include Asciidoc Preview package for Atom: https://atom.io/packages/asciidoc-preview to help beginners check their changes are correct
4. Include an explanation of what a build file is (for beginners) & how to use it for this project, especially if they don't use the Atom Asciidoc Preview package|

[eddiejaoude]

Hows does this look @charlottebrf ?