Update CONTRIBUTING.md about new versions

Author: eregon

CONTRIBUTING.md

@@ -22,6 +22,5 @@ $ cp pre-commit .git/hooks/pre-commit
 
 ## Adding a new Ruby version
 
-Add the new version in `ruby-builder-versions.json`,
-then follow the steps above in [Regenerating dist/index.js](#regenerating-distindexjs) to update `dist/index.js`.
-Finally, update the "Supported Versions" section of the README if needed.
+This is entirely automated now since [this issue](https://github.com/ruby/setup-ruby/issues/254).
+If you do not see a new version more than 24 hours after it was released feel free to file an issue.