Niner since 2012
https://aka.ms/uwpgamedevguide is a great starting point.
@Ken: Thanks, we think it's pretty darned awesome too! The US has certainly not been excluded, we're just finalizing some of the finer details before we publish their locations.
Yesterday we put up more countries, and today another batch is coming. We're expecting to add them on a daily basis over the next couple of weeks, so keep an eye out for US locations.