Colby Goettel

ruby-shadow has been updated (as of today) to 2.5.1 which allows it to build on Ruby 3.0.0 so that's no longer a blocking issue.

`ruby extconf.rb` works on Ruby 3.0.0. Not a full test, but a good first step.

I think the README currently says that this is out of scope. Either way, update the documentation.