LGTM except for

>+    (license (license:non-copyleft "file://readme.txt"))))

That is an invalid URL.

Should be:

+    (license (license:non-copyleft "file:///readme.txt"))))

There are cases where you want to refer to files on a specific host
(I've actually used file manager programs which support drag&drop 
across hosts--it's nice).

See also https://tools.ietf.org/html/rfc8089 for possible forms.

What is also valid by now is "file:readme.txt".