Add codeowners file

[nlhkabu]

Allows github to automatically assign reviewers based on ownership areas

Example from CPython:
https://github.com/python/cpython/blob/master/.github/CODEOWNERS

[nlhkabu]

I should own / co-own:

• README.rst
• docs
• warehouse/templates
• warehouse/static/html
• warehouse/static/sass
• warehouse/static/images
• warehouse/admin/templates
• warehouse/admin/static/css
• warehouse/admin/static/fonts
• warehouse/admin/static/img
• warehouse/admin/static/webfonts

[di]

@nlhkabu, I agree we should add a CODEOWNERS for you for those files, but I don't think we should add it for the rest of us. I personally tend to treat a pending review as a blocker on merging a PR, and would prefer to keep the "request review" feature as a stronger signal, e.g. "I actually need you specifically to look at this PR."

[nlhkabu]

Thanks for your feedback @di. I have been thinking about this -> I don't want to become a blocker to merging a PR if I am unavailable for a time period, so I'm going to close this issue.

In the time I've been working on the project, I can think of very few times I've not been added to review a design/HTML/CSS change, so I am not sure there is a real problem here :)