asciidoctor-diagram might benefit from additional installation tooling

[marmalodak]

I'm grateful to the people who work on asciidoc, asciidoctor, and asciidoctor-diagram. I want to express support and affirmation to the people who work on these projects and also to the people that field bug reports and diligently read and triage them. I understand that the work can be challenging and might sometimes be unappreciated. Please accept gratitude and respect.

This is my experience leading up to me filing this issue.

1. Unadorned GEM is not supported by asiidoctor, "use rvm or bundler"
2. rvm failed for me under fedora
3. While I've succeeded with rvm in the past, today I also struggled with rvm on macos
4. I took the advice from brew-gem for macos and struggled: see this issue
5. When using bundle, asciidoctor-diagram cannot find java

I've spent some time with Python and no time with Ruby which leaves me at a disadvantage when trying to understand how to satisfy Ruby's installation requirements.