Add CODEOWNERS to github repository generated by 'summoner'

[chshersh]

Recent GHC introduced CODEOWNERS feature. It would be great to generate some default file for github repositories.

Example: https://github.com/serokell/importify/blob/master/CODEOWNERS

Announce: https://github.com/blog/2392-introducing-code-owners

[luvhalvorson]

Wasn't sure what you want here. I made a PR anyway, for Hacktoberfest is about to end. might look kinda silly tho. Would like to improve it if you can provide details :)

[vrom911]

@chshersh thanks for this wonderful idea! It's definitely worth it to have it!
@luvhalvorson I made some comments in your PR, thank you :)

[chshersh]

Turned out it's not useful to have CODEOWNERS until we figure out how to support user vs. organization.

[vrom911]

A subtask of #41. Better to keep all of such kind in one place.
(Also it's not clear how to handle this one)