Add copyright notice to license

[ghost]

This is a great starter but as it stands the work added to the gatsby-starter-default is unclear from a licensing perspective given no copyright notice was added by the author. Please add a copyright notice. For more information see: https://writing.kemitchell.com/2016/09/21/MIT-License-Line-by-Line.html. If you're unsure how to add a notice let me know and I'll help.

Related: gatsbyjs/gatsby#25091