Add CODEOWNERS

[batpigandme]

Hi @smbache,

A heads up: at this past RStudio work week, we decided to try to formally implement "understudies" for all the packages (i.e. having a second person to help increase our code reviews, etc. — in this case, @lionel-). I'll submit a PR with a CODEOWNERS file. The link in it currently goes to nothing, but will ultimately go to a description of this process for the tidyverse.org page development practices).

Thanks,
Mara