yrbss documentation

Do we know which year's survey is included in this dataset? Also, do we know if the variable called `gender` is what's identified in the [2017 data documentation](https://www.cdc.gov/healthyyouth/data/yrbs/pdf/2017/2017_yrbs_sadc_documentation.pdf) as `sex`?

I'm happy to do a PR to clarify those things if we can track them down.