add CODEOWNERS file to indicate who has maintainer authority in this repo

[bpholt]

I thought it'd be helpful to have this in place in case new contributors want to tag us on PRs, etc. (I have a couple open PRs in places where I don't know who to tag to get their attention, so I thought it'd be nice to do that here.)

[codecov-commenter]

Codecov Report

Merging #524 (c4ba820) into main (1043ccd) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #524   +/-   ##
=======================================
  Coverage   90.60%   90.60%           
=======================================
  Files          18       18           
  Lines         298      298           
  Branches        2        2           
=======================================
  Hits          270      270           
  Misses         28       28           

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