Document decision points in the WNV phylogenetic build

[DOH-LMT2303]

Possible solution

Add information to the README file about the decisions points that were made to build the global and the local WNV builds:

• Selection of roots
• Subsampling
• Lineage calling in Nextclade and coloring in Auspice

Then, select what points to highlight in the Description section of the UI (description.md)