Document how to add a new Ruby version

Author: eregon

CONTRIBUTING.md

@@ -19,3 +19,7 @@ It is recommended to add this as a `git` `pre-commit` hook:
 ```bash
 $ cp pre-commit .git/hooks/pre-commit
 ```
+
+## Adding a new Ruby version
+
+Add the new version in `ruby-builder-versions.js` and update the README.