is_working_gutenberg_mirror: Check if an http(s) or ftp(s) url resolves to a working Gutenberg mirror.
Description
Checks for a root level README file at url with reference to
GUTINDEX.ALL. If this exists, url is most likely a working
Gutenberg mirror.
Usage
is_working_gutenberg_mirror(url)
Value
Boolean: whether the url resolves to a mirror.