Proposed additional link to copyright guidance

As identified by @tvol in #20 after @noahkunin's question, adds a link to the [Wikisource excerpt](https://en.wikisource.org/wiki/Page:H.R._Rep._No._94-1476_(1976)_Page_059.djvu) that makes it much more obvious what the original language around foreign copyrightability is.

I didn't replace the original link because this is Wikisource and not the primary material. Open to other ideas of how to display this.

Since this is a non-substantive change, I'm proposing this to the main `gh-pages` branch, which, if merged, will appear on the site immediately.