[Spike] Review documentation for registry stacks using parent devfiles

[thepetk]

/area documentation

Which area is this documentation change related to?

/area registry

Issue details

After introducing new stacks that are using a parent devfile (first it was seen here: devfile/registry#401), it is necessary to review our repo documentation (contributing guides, etc) in order to make sure that we are covering this case.

For example, we should explicitly mention that the parent devfile should be public as it will be a resource of the community registry.

What mistake did you find / what is missing in the documentation?
N/A

Acceptance Criteria

• The repo documentation is up-to-date containing sufficient info for registry stacks using parent devfiles