Clarify GDK's responsibilities

What does this Merge Request do and why?

This MR updates the README.md to make it more clear as to the types of projects, software and services the GDK is responsible for managing.

Related: !2528 (merged)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.