docs: Start defining the owners of code sections

When filing a new merge request it's often hard to know who to ask for a review; using the Git log doesn't always help — the person that touched a file last may just be fixing the build or a compiler warning.

The CODE-OWNERS file format is something that GitHub uses in order to pre-fill the list of reviewers:

https://help.github.com/articles/about-codeowners/

Ideally, in the future, we'll be able to use this file with a bot like homu to automatically go through newly filed merge requests and automatically ask the relevant people for reviews, instead of doing this manually.