Update README.md (#287)
Update README.md (#287)

To contributors
In February 2021, IRuby became the canonical repository and is no longer a fork from minrk/iruby. Please fork from this repository again before making pull requests. Thank you for contributing to IRuby.
SciRuby/iruby,SciRuby/iruby,SciRuby/iruby,SciRuby/iruby