Add the dependency to `rouge` in the gemspec file

[tperami]

Add something like

s.add_runtime_dependency 'rouge'

To the gemspec file. I don't exactly how it works

[Timmmm]

I don't think this repo does depend on rouge though. It's only used if the asciidoc file has its highlighter set to rouge.

The riscv-isa-manual now uses rouge and asciidoctor-sail for all outputs.