Add attribution for dir-reading code in license

[hartogss]

However, http://www.martinbroadhurst.com/list-the-files-in-a-directory-in-c.html
seems to be unreachable currently

[hartogss]

Found snapshot of site on waybackmachine, however no copyright notice to be found.

[hartogss]

Other option: move to C++17 std::filesystem.