Add a development guide on how to setup a Ruby development environment

[postmodern]

Add a guide to /docs/ on how to setup a Ruby development environment.

• Recommend using the ronin-dev.sh script.
• Include manually instructions on how to install ruby from the package manager:
  • Mention the differences between ruby, ruby-dev, and ruby-full.
  • Also mention that one must install libyaml-dev on Ubuntu in order for the psych gem to be compiled by bundle install.
• Cover chruby and ruby-install as a more advanced option.
• Also mention rbenv and ruby-build.