Mention Perl dependency for Linux

[GeoffreyBooth]

I was using this in a Docker Alpine image and got this error:

throw er; // Unhandled 'error' event
^
Error: spawn /opt/bundle/bundle/programs/server/npm/node_modules/exiftool-vendored.pl/bin/exiftool ENOENT

The error was exiftool failing to start:

exiftool: /usr/bin/perl: bad interpreter: No such file or directory

Because Perl was missing. Alpine Linux aims to be as small as possible, and as such it doesn’t come with Perl installed. One needs to install it (apk add perl is all it takes).

This isn’t really particular to Alpine though, as anyone might happen to be in an environment that lacks Perl. You might want to mention in the README and/or the wiki that Perl is a dependency that won’t be installed by this package, that the user is responsible for ensuring is available in the environment. I see that the README mentions that Perl will be installed for Windows environments, but presumably that doesn’t happen for Linux.