Add: documentation about updating contributor names in the contributors.yml file only

[lwasser]

          Update: i'll open an issue based on this comment. 

Update the Contributing file for this repo (or the readme) to explain that we should ONLY update a person's name in the contributors.yml file. If we try to update the packages.yml file with the name it will be overwritten. This is because our workflow treats the contributors.yml file as the singel source of truth and the single plae to manage people. The names listed here assume that we can gather metadata based on a contributor's GitHub username from the contributor file.

takeaway: update names in the contributors.yml file always (and document this well)

Originally posted by @lwasser in #527 (comment)