Add contributors guide how to run a single test

[ahus1]

When preparing PR #358, I struggled with running a single test of the big test suite locally.

This is probably due to me being a Ruby newbie. I tried to apply some things about running a single test from the Asciidoctor contributor's guide, but did work out. As a last resort I delete the tests I didn't want to run locally, and reverted that change before pushing the change.

I'd love to see a chapter in this project's contributors guide for this project about how to run a single test.

[pepijnve]

I usually do this from my IDE (RubyMine). From a shell it's a standard RSpec invocation. See below for instance:

pepijn@Cerris asciidoctor-diagram % bundle exec rspec spec/plantuml_spec.rb -e "plantuml should generate image blocks when format is set to 'png'"
Run options:
  include {:full_description=>/plantuml\ should\ generate\ image\ blocks\ when\ format\ is\ set\ to\ 'png'/}
  exclude {:broken_on_osx=>true}

Asciidoctor::Diagram::PlantUmlBlockMacroProcessor
  plantuml should generate image blocks when format is set to 'png'

Asciidoctor::Diagram::PlantUmlBlockProcessor
  plantuml should generate image blocks when format is set to 'png'

Finished in 1.64 seconds (files took 0.27539 seconds to load)
2 examples, 0 failures

[pepijnve]

See https://relishapp.com/rspec/rspec-core/v/3-10/docs/command-line/example-option for more detailed information.

[ahus1]

Thanks, I'll use the suggested scripts and links next time.