Add CODEOWNERS file

I thought it might be worth requiring approvals on this project. The best way to achieve that (I think) is a CODEOWNERS file (https://docs.gitlab.com/ee/user/project/codeowners/index.html).

The intent is that:

• Most things would require Technical Writing Leadership to approved.
• Code things could be worked on without needing Technical Writing Leadership to approve.
• Suzanne is both Technical Writing Leadership and sometimes works with Sara on some of the reports, so Suzanne can approve either type of things.

WDYT?