Add CODEOWNERS file

[jameslamb]

GitHub allows you to add a CODEOWNERS file that will be used to set default reviewers when PRs are opened.

This has to be done by a repo admin, so assigning to myself.

We should do this so we automatically are added as reviewers when people submit PRs!

This can automate things that would otherwise only be known by convention, like "person X owns the R code, person Y owns the flask app"