Update templates to include header in the README

What does this MR do?

Add updated export files for project templates. These include a header in the README to warn them that the project was imported.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• [-] Conform by the merge request performance guides
• [-] Conform by the style guides
• [-] Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#36904

/cc @marin @briann @bikebilly