Provide a clearer explanation for the subsampling approach we use

[huddlej]

Elaborate on the purpose of subsampling in the results section when we first describe the available sequences. The main purposes are to:

• minimize regional bias in sequence availability by evenly sampling across all regions
• maintain reasonable run times for real-time phylogenetic inference
• keep the subsampling in this study the same as our original implementation in Huddleston et al. 2020 and minimize the number of variables we changed between studies