Add info about asdf error on Ubuntu

Mike Jangrequested to merge
mjang-update-gdk-ubuntu-for-asdf into master

What does this Merge Request do?

Add info on an asdf error message, when installing on Ubuntu 18.04

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.
Edited by Mike Jang

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