Add Meghan to AUTHORS.md

[seisman]

@meghanrjones Don't forget to add yourself to AUTHORS.md

[maxrjones]

Thanks for the note, @seisman! I plan to address this at the same time as #2330, using the format posted in #2330 (comment) (i.e., adding a team gallery with information about the addition process and converting AUTHORS.md to thanks.md).