support project.el external roots

[fargiolas]

Maybe the feature is already there and I'm not aware of it.

It would be great if rg.el could extend search to multiple directories, especially when a project uses the external roots feature from project.el.

See e.g. what you can do with project-or-external-find-regexp (C-x-p-G) that calls xref-search-program on both project root and external roots.

[dajva]

I have been opting not to include such functionality since it would interfere with one of the main assumptions of the package's UX design, iow do quick simple searches and refine as you go from results buffer with simple key presses.
With that said, I think it would be great to support this in some way but I have not figured out a way to include it smoothly and play well with other features. There is the full command line search that should work for this but that essentially exclude the refine functionality of the results buffer. I do miss these kinds of advanced searches myself sometimes though.