Provide transition to asdf instructions

What does this Merge Request do?

Provides information for those that want to transition away from managing their own dependencies, to use asdf with GDK.

CC @ashmckenzie @stanhu

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.