docs: Start defining the owners of code sections

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.