Add CODEOWNERS file

[cbeams]

Per documentation at https://help.github.com/articles/about-codeowners/, this file specifies who should get requested to review what in the repository. I was prompted to add this by the desire to auto-request @blabno for all /assets PRs, and added other obvious or established code owners as well.

@ManfredKarrer, take note that while you're the default code owner, and will therefore be requested to review all PRs by default, you will not be requested to review any PRs for subsequent entries in the file that you are not the code owner for. So for example, you would be requested to review anything under /core because no more specific code owner is specified for that, but you will not be requested to review *.gradle files, because I'm the code owner for those.

[ManfredKarrer]

utACK

Looks good to me

[devinbileck]

@cbeams As you may have noticed, the latest PRs for assets do not have @blabno auto-assigned as reviewer. It appears that auto-assigning code reviewers only works if the owner has push access to the repo. As per https://help.github.com/articles/repository-permission-levels-for-an-organization/, to Act as a designated code owner for a repository requires write permissions.

[cbeams]

Good point, I hadn't realized that. Even though it wasn't the intended result, the outcome is actually OK, because while @blabno doesn't get assigned as a reviewer, neither does anybody else (usually @ManfredKarrer is usually the fallback as he's the catch-all * code owner). @blabno can still review these as he has time, but it's basically a good thing that no one is explicitly "on the hook" for these reviews. We just get to them as we have time.