initial commit: rough out a server-side php endpoint
This is mostly just Composer & Slim framework boilerplate. There's a bit
of actual code in src/routes.php which uses davechild/textstatistics to
assign Dale-Chall readability scores to page intro extracts.
The need for a framework here at all is, of course, pretty questionable,
but then maybe we already know the initial use case is going to elaborate