Update Readme to include "select multiple figma items."

[JeremyTheModernist]

Update Readme

Hey Brian, via our conversation on twitter. I'm wondering if it would be valuable to update the README to contain another line item at the end, that reads something to effect of:

Select multiple items in Figma with the appropriate layer naming, eg {__fieldname}, and then choose your Github Data Source from the modal.

If this seems fitting, I'm happy to file a PR. Thanks!