Document how to apply Ruby patches

What does this Merge Request do?

Related to gitlab#296530 (closed).

This documents how to patch GDK when using asdf to apply custom Ruby patches.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.